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Add missing LICENSE.

From README:

RPy2 is subject to the Mozilla Public License Version 1.1 (the
"License"); you may not use RPy2 except in compliance with the
License. You may obtain a copy of the License at
http://www.mozilla.org/MPL/ (or see the file MPL_LICENSE)

Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and
limitations under the License.

The Original Code is the RPy2 python package, itself subdivived
into subpackages and modules.

All code is Copyright of the respective author(s).
See the file AUTHORS for further details regarding authorship.

Alternatively, RPy2 may be used under the terms of either the GNU
General Public License Version 2 or later (the "GPL", see the file
GPL_LICENSE), or the GNU Lesser General Public License Version 2.1
or later (the "LGPL", see the file LGPL_LICENSE), in which case the
provisions of the GPL or the LGPL are applicable instead of those
above. If you wish to allow use of your version of this file only
under the terms of either the GPL or the LGPL, and not to allow
others to use your version of this file under the terms of the MPL,
indicate your decision by deleting the provisions above and replace
them with the notice and other provisions required by the GPL or the
LGPL. If you do not delete the provisions above, a recipient may use
your version of this file under the terms of any one of the MPL, the
GPL or the LGPL.