Merge pull request #2 from lvh/contributing

Simple CONTRIBUTING file, refs PEPs 8, 257

CONTRIBUTING.rst

@@ -0,0 +1,21 @@
+This repository has a mandatory code review policy. Contributions
+should happen through pull requests. Never commit to ``master``
+directly.
+
+When in doubt, refer to PEP 8 for Python code.
+
+Write docstrings like this::
+
+  def some_function(some_arg):
+      """
+      Does some things.
+
+      :param some_arg: Some argument.
+      """
+
+So, specifically:
+
+- Always use three double quotes.
+- Put the three double quotes on their own line.
+- No blank line at the end.
+- Use Sphinx parameter/attribute documentation syntax.