SubmittingPatches: Document some extra tags used in commit messages

SubmittingPatches: Document some extra tags used in commit messages

Document the meanings of the tags "Reported-by:", "Acked-by:",
"Reviewed-by:" and "Tested-by:" clearly. Also mention that the user is
free to use any custom tags.

Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Liked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
[wking@tremily.us: removed diff context to merge with Linux kernel
version]
Signed-off-by: W. Trevor King <wking@tremily.us>