SubmittingPatches: add style recommendation to use imperative descriptions

Most commit messages use this style, and the recommendation frequently
comes up in discussions (especially in response to patches that don't
use it), but that recommendation doesn't actually appear anywhere in
Documentation.  Add this style guideline to SubmittingPatches, using the
description from git's SubmittingPatches.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Borislav Petkov <bp@suse.de>
Cc: Rob Landley <rob@landley.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/SubmittingPatches

@@ -106,6 +106,11 @@ I.e., the patch (series) and its description should be self-contained.
 This benefits both the patch merger(s) and reviewers.  Some reviewers
 probably didn't even receive earlier versions of the patch.
 
+Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
+instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
+to do frotz", as if you are giving orders to the codebase to change
+its behaviour.
+
 If the patch fixes a logged bug entry, refer to that bug entry by
 number and URL.