Add contribution instructions in CONTRIBUTING.rst

Closes https://github.com/adrienverge/yamllint/issues/263.

CONTRIBUTING.rst

@@ -0,0 +1,44 @@
+Contributing
+============
+
+Pull requests are the best way to propose changes to the codebase.
+Contributions are welcome, but they have to meet some criteria.
+
+Pull Request Process
+--------------------
+
+1. Fork this Git repository and create your branch from ``master``.
+
+2. Make sure the tests pass:
+
+   .. code:: bash
+
+    python setup.py test
+
+3. If you add code that should be tested, add tests.
+
+4. Make sure the linters pass:
+
+   .. code:: bash
+
+    flake8 .
+
+   If you added/modified documentation:
+
+   .. code:: bash
+
+    doc8 $(git ls-files '*.rst')
+
+   If you touched YAML files:
+
+   .. code:: bash
+
+    yamllint --strict $(git ls-files '*.yaml' '*.yml')
+
+5. If relevant, update documentation (either in ``docs`` directly or in rules
+   files themselves).
+
+6. Write a `good commit message
+   <http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html>`_.
+
+7. Then, open a pull request.