Documentation: user-manual: add information about "git help" at the beginning

Talking about "git help" is useful because it has a few more
features (like when using it without arguments or with "-a") and
it may work on non unix like platforms.
Signed-off-by: NChristian Couder <chriscool@tuxfamily.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -18,12 +18,22 @@ People needing to do actual development will also want to read
 Further chapters cover more specialized topics.
 
 Comprehensive reference documentation is available through the man
-pages.  For a command such as "git clone <repo>", just use
+pages, or linkgit:git-help[1] command.  For example, for the command
+"git clone <repo>", you can either use:
 
 ------------------------------------------------
 $ man git-clone
 ------------------------------------------------
 
+or:
+
+------------------------------------------------
+$ git help clone
+------------------------------------------------
+
+With the latter, you can use the manual viewer of your choice; see
+linkgit:git-help[1] for more information.
+
 See also <<git-quick-start>> for a brief overview of git commands,
 without any explanation.