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>

Documentation/SubmittingPatches

@@ -264,12 +264,21 @@ the change to its true author (see (2) above).
 Also notice that a real name is used in the Signed-off-by: line. Please
 don't hide your real name.
 
-Some people also put extra tags at the end.
-
-"Acked-by:" says that the patch was reviewed by the person who
-is more familiar with the issues and the area the patch attempts
-to modify.  "Tested-by:" says the patch was tested by the person
-and found to have the desired effect.
+If you like, you can put extra tags at the end:
+
+1. "Reported-by:" is used to to credit someone who found the bug that
+   the patch attempts to fix.
+2. "Acked-by:" says that the person who is more familiar with the area
+   the patch attempts to modify liked the patch.
+3. "Reviewed-by:", unlike the other tags, can only be offered by the
+   reviewer and means that she is completely satisfied that the patch
+   is ready for application.  It is usually offered only after a
+   detailed review.
+4. "Tested-by:" is used to indicate that the person applied the patch
+   and found it to have the desired effect.
+
+You can also create your own tag or use one that's in common usage
+such as "Thanks-to:", "Based-on-patch-by:", or "Mentored-by:".
 
 ------------------------------------------------
 An ideal patch flow