Merge pull request #56 from justquick/master

Add the missing README.rst to MANIFEST.in

(it seems like generated tarballs/zipballs already included README.rst,
perhaps due to some other fix landed since this branch was created, but
it's a good idea to include it in MANIFEST.in anyways)

refs #56

MANIFEST.in

 include tox.ini
 include LICENSE
+include README.rst
 
 # libsodium files
 recursive-include src/libsodium *