updated comment in .mailmap file

Author: cebe

.mailmap

@@ -1,4 +1,6 @@
-# This files is used to fix authors email addresses 
+# This file is used to fix authors email addresses in old svn commit history 
+# See git docs for details:
+# http://www.kernel.org/pub/software/scm/git/docs/git-shortlog.html#_mapping_authors
 
 Qiang Xue <qiang.xue@gmail.com> qiang.xue <qiang.xue@c7f931c7-7552-0410-b027-2fb3d89d9100>
 Qiang Xue <qiang.xue@gmail.com> qiang.xue@gmail.com <qiang.xue@gmail.com@c7f931c7-7552-0410-b027-2fb3d89d9100>