get_maintainer: add missing documentation for --git-blame-signatures

I really haven't used this option much myself, so feel free to improve on
the documentation for it.  I just noticed it while inspecting this script
for undocumented features.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/get_maintainer.pl

@@ -781,6 +781,7 @@ MAINTAINER field selection options:
     --git-max-maintainers => maximum maintainers to add (default: $email_git_max_maintainers)
     --git-min-percent => minimum percentage of commits required (default: $email_git_min_percent)
     --git-blame => use git blame to find modified commits for patch or file
+    --git-blame-signatures => when used with --git-blame, also include all commit signers
     --git-since => git history to use (default: $email_git_since)
     --hg-since => hg history to use (default: $email_hg_since)
     --interactive => display a menu (mostly useful if used with the --git option)