DOC: Add six copyright (#375)

Author: Ben Cipollini

COPYING

@@ -209,3 +209,32 @@ the PDDL version 1.0 available at http://opendatacommons.org/licenses/pddl/1.0/
 
 is courtesy of the University of Massachusetts Medical School, also released
 under the PDDL.
+
+
+Six
+--------------------
+
+In ``nibabel/externals/six.py``
+
+Copied from: https://pypi.python.org/packages/source/s/six/six-1.3.0.tar.gz#md5=ec47fe6070a8a64c802363d2c2b1e2ee
+
+    Copyright (c) 2010-2013 Benjamin Peterson
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy of
+    this software and associated documentation files (the "Software"), to deal in
+    the Software without restriction, including without limitation the rights to
+    use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+    the Software, and to permit persons to whom the Software is furnished to do so,
+    subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in all
+    copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+    FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+    COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+    IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+    CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+