Mention exceptions to Google Style Guide in SUBMITTING_PATCHES.md

Change-Id: I05d313c66f312942405a884a54118cb1d7af1bac
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254671
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>