Added CONTRIBUTING.rst

CONTRIBUTING.rst

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+Contributing to fbchat
+======================
+
+Thanks for reading this, all contributions are very much welcome!
+
+Please be aware that ``fbchat`` uses `Scemantic Versioning <https://semver.org/>`__
+That means that if you're submitting a breaking change, it will probably take a while before it gets considered.
+
+In that case, you can point your PR to the ``2.0.0-dev`` branch, where the API is being properly developed.
+Otherwise, just point it to ``master``.
+
+Testing Environment
+-------------------
+
+The tests use `pytest <https://docs.pytest.org/>`__, and to work they need two Facebook accounts, and a group thread between these.
+To set these up, you should export the following environment variables:
+
+``client1_email``, ``client1_password``, ``client2_email``, ``client2_password`` and ``group_id``
+
+If you're not able to do this, consider simply running ``pytest -m offline``.
+
+And if you're adding new functionality, if possible, make sure to create a new test for it.

MANIFEST.in

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 include LICENSE
+include CONTRIBUTING.rst
 include README.rst