Documentation: add instructions to help setup gmail 2FA

For those who use two-factor authentication with gmail, git-send-email
will not work unless it is setup with an app-specific password. The
example for setting up git-send-email for use with gmail will now
include information on generating and storing the app-specific password.
Signed-off-by: NMichael Rappazzo <rappazzo@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -450,6 +450,19 @@ edit ~/.gitconfig to specify your account settings:
 		smtpUser = yourname@gmail.com
 		smtpServerPort = 587
 
+If you have multifactor authentication setup on your gmail acocunt, you will
+need to generate an app-specific password for use with 'git send-email'. Visit
+https://security.google.com/settings/security/apppasswords to setup an
+app-specific password.  Once setup, you can store it with the credentials
+helper:
+
+	$ git credential fill
+	protocol=smtp
+	host=smtp.gmail.com
+	username=youname@gmail.com
+	password=app-password
+
+
 Once your commits are ready to be sent to the mailing list, run the
 following commands: