doc: about email clients for Linux patches

Requested by Jeff Garzik.
v3, updated from lkml comments.

Add info about various email clients and their applicability
in being used to send Linux kernel patches.

Some notes takes from http://mbligh.org/linuxdocs/Email/Clients
Portions used with permission.
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Cc: Jeff Garzik <jeff@garzik.org>
Cc: Martin Bligh <mbligh@mbligh.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/email-clients.txt

+Email clients info for Linux
+======================================================================
+
+General Preferences
+----------------------------------------------------------------------
+Patches for the Linux kernel are submitted via email, preferably as
+inline text in the body of the email.  Some maintainers accept
+attachments, but then the attachments should have content-type
+"text/plain".  However, attachments are generally frowned upon because
+it makes quoting portions of the patch more difficult in the patch
+review process.
+
+Email clients that are used for Linux kernel patches should send the
+patch text untouched.  For example, they should not modify or delete tabs
+or spaces, even at the beginning or end of lines.
+
+Don't send patches with "format=flowed".  This can cause unexpected
+and unwanted line breaks.
+
+Don't let your email client do automatic word wrapping for you.
+This can also corrupt your patch.
+
+Email clients should not modify the character set encoding of the text.
+Emailed patches should be in ASCII or UTF-8 encoding only.
+If you configure your email client to send emails with UTF-8 encoding,
+you avoid some possible charset problems.
+
+Email clients should generate and maintain References: or In-Reply-To:
+headers so that mail threading is not broken.
+
+Copy-and-paste (or cut-and-paste) usually does not work for patches
+because tabs are converted to spaces.  Using xclipboard, xclip, and/or
+xcutsel may work, but it's best to test this for yourself or just avoid
+copy-and-paste.
+
+Don't use PGP/GPG signatures in mail that contains patches.
+This breaks many scripts that read and apply the patches.
+(This should be fixable.)
+
+It's a good idea to send a patch to yourself, save the received message,
+and successfully apply it with 'patch' before sending patches to Linux
+mailing lists.
+
+
+Some email client (MUA) hints
+----------------------------------------------------------------------
+Here are some specific MUA configuration hints for editing and sending
+patches for the Linux kernel.  These are not meant to be complete
+software package configuration summaries.
+
+Legend:
+TUI = text-based user interface
+GUI = graphical user interface
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Alpine (TUI)
+
+Config options:
+In the "Sending Preferences" section:
+
+- "Do Not Send Flowed Text" must be enabled
+- "Strip Whitespace Before Sending" must be disabled
+
+When composing the message, the cursor should be placed where the patch
+should appear, and then pressing CTRL-R let you specify the patch file
+to insert into the message.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Evolution (GUI)
+
+Some people use this successfully for patches.
+
+When composing mail select: Preformat
+  from Format->Heading->Preformatted (Ctrl-7)
+  or the toolbar
+
+Then use:
+  Insert->Text File... (Alt-n x)
+to insert the patch.
+
+You can also "diff -Nru old.c new.c | xclip", select Preformat, then
+paste with the middle button.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Kmail (GUI)
+
+Some people use Kmail successfully for patches.
+
+The default setting of not composing in HTML is appropriate; do not
+enable it.
+
+When composing an email, under options, uncheck "word wrap". The only
+disadvantage is any text you type in the email will not be word-wrapped
+so you will have to manually word wrap text before the patch. The easiest
+way around this is to compose your email with word wrap enabled, then save
+it as a draft. Once you pull it up again from your drafts it is now hard
+word-wrapped and you can uncheck "word wrap" without losing the existing
+wrapping.
+
+At the bottom of your email, put the commonly-used patch delimiter before
+inserting your patch:  three hyphens (---).
+
+Then from the "Message" menu item, select insert file and choose your patch.
+As an added bonus you can customise the message creation toolbar menu
+and put the "insert file" icon there.
+
+You can safely GPG sign attachments, but inlined text is preferred for
+patches so do not GPG sign them.  Signing patches that have been inserted
+as inlined text will make them tricky to extract from their 7-bit encoding.
+
+If you absolutely must send patches as attachments instead of inlining
+them as text, right click on the attachment and select properties, and
+highlight "Suggest automatic display" to make the attachment inlined to
+make it more viewable.
+
+When saving patches that are sent as inlined text, select the email that
+contains the patch from the message list pane, right click and select
+"save as".  You can use the whole email unmodified as a patch if it was
+properly composed.  There is no option currently to save the email when you
+are actually viewing it in its own window -- there has been a request filed
+at kmail's bugzilla and hopefully this will be addressed.  Emails are saved
+as read-write for user only so you will have to chmod them to make them
+group and world readable if you copy them elsewhere.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Lotus Notes (GUI)
+
+Run away from it.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Mutt (TUI)
+
+Plenty of Linux developers use mutt, so it must work pretty well.
+
+Mutt doesn't come with an editor, so whatever editor you use should be
+used in a way that there are no automatic linebreaks.  Most editors have
+an "insert file" option that inserts the contents of a file unaltered.
+
+To use 'vim' with mutt:
+  set editor="vi"
+
+  If using xclip, type the command
+  :set paste
+  before middle button or shift-insert or use
+  :r filename
+
+if you want to include the patch inline.
+(a)ttach works fine without "set paste".
+
+Config options:
+It should work with default settings.
+However, it's a good idea to set the "send_charset" to:
+  set send_charset="us-ascii:utf-8"
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Pine (TUI)
+
+Pine has had some whitespace truncation issues in the past, but these
+should all be fixed now.
+
+Use alpine (pine's successor) if you can.
+
+Config options:
+- quell-flowed-text is needed for recent versions
+- the "no-strip-whitespace-before-send" option is needed
+
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Sylpheed (GUI)
+
+- Works well for inlining text (or using attachments).
+- Allows use of an external editor.
+- Not good for IMAP.
+- Is slow on large folders.
+- Won't do TLS SMTP auth over a non-SSL connection.
+- Has a helpful ruler bar in the compose window.
+- Adding addresses to address book doesn't understand the display name
+  properly.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Thunderbird (GUI)
+
+By default, thunderbird likes to mangle text, but there are ways to
+coerce it into being nice.
+
+- Under account settings, composition and addressing, uncheck "Compose
+  messages in HTML format".
+
+- Edit your Thunderbird config settings to tell it not to wrap lines:
+      user_pref("mailnews.wraplength", 0);
+
+- Edit your Thunderbird config settings so that it won't use format=flowed:
+      user_pref("mailnews.send_plaintext_flowed", false);
+
+- You need to get Thunderbird into preformat mode:
+. If you compose HTML messages by default, it's not too hard. Just select
+  "Preformat" from the drop-down box just under the subject line.
+. If you compose in text by default, you have to tell it to compose a new
+  message in HTML (just as a one-off), and then force it from there back to
+  text, else it will wrap lines. To do this, use shift-click on the Write
+  icon to compose to get HTML compose mode, then select "Preformat" from
+  the drop-down box just under the subject line.
+
+- Allows use of an external editor:
+  The easiest thing to do with Thunderbird and patches is to use an
+  "external editor" extension and then just use your favorite $EDITOR
+  for reading/merging patches into the body text.  To do this, download
+  and install the extension, then add a button for it using
+  View->Toolbars->Customize... and finally just click on it when in the
+  Compose dialog.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+TkRat (GUI)
+
+Works.  Use "Insert file..." or external editor.
+
+                                ###