Mention sortable.js license (MIT) in doc/license.

Thanks-to: Stephen Gordon <stephen.a.gordon@gmail.com>

doc/license

-License is GPLv2/GPLv3, see doc/GPLv2 and doc/GPLv3 respectively.
+gitstats license is GPLv2/GPLv3, see doc/GPLv2 and doc/GPLv3 respectively.
+
+sortable.js, contained in gitstats, is licensed under the MIT license. See the
+file itself for details.