Document --check option to git diff.

Signed-off-by: Bill Lear <rael@zopyra.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/SubmittingPatches

@@ -23,7 +23,8 @@ probably need to split up your commit to finer grained pieces.
 
 Oh, another thing.  I am picky about whitespaces.  Make sure your
 changes do not trigger errors with the sample pre-commit hook shipped
-in templates/hooks--pre-commit.
+in templates/hooks--pre-commit.  To help ensure this does not happen,
+run git diff --check on your changes before you commit.
 
 
 (2) Generate your patch using git tools out of your commits.

Documentation/diff-options.txt

@@ -58,6 +58,10 @@
 	Turn off rename detection, even when the configuration
 	file gives the default to do so.
 
+--check::
+	Warn if changes introduce trailing whitespace
+	or an indent that uses a space before a tab.
+
 --full-index::
 	Instead of the first handful characters, show full
 	object name of pre- and post-image blob on the "index"