Added comment to explain .mailmap file and its format.

.mailmap

+# Prevent git from showing duplicate names with commands like "git shortlog"
+# # See the manpage of git-shortlog for details.
+# # The syntax is:
+# # Name that should be used <email that should be used> Bad name <bad email>
+# #
+# # You can skip Bad name if it is the same as the one that should be used, and is unique.
+# #
+# # This file is up-to-date if the command git log --format="%aN <%aE>" | sort -u
+# # gives no duplicates.
+
 <abergeron@gmail.com> <anakha@kami.(none)>
 David Warde-Farley <wardefar@iro.umontreal.ca> David Warde-Farley <dwf@cs.toronto.edu>
 David Warde-Farley <wardefar@iro.umontreal.ca> David Warde Farley <dwf@cs.toronto.edu>