CONTRIBUTING: Add skeleton.

There is much more to mention, like adding tests to our test suite and so
on.

Also, the formatting sucks.

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+Please, before sending patches, it is helpful if you:
+
+* Make sure that you have all dependencies installed, like via
+
+        pip install -r requirements
+
+* Run the test suite with
+
+        nosetests --verbose --with-coverage --cover-package=coursera
+
+* Check for any potential bugs with the help of [`pep8`][pep8],
+  [`pyflakes`][pyflakes], and [`pylint`][pylint].
+
+[pep8]: https://pypi.python.org/pypi/pep8
+[pyflakes]: https://pypi.python.org/pypi/pyflakes/
+[pylint]: https://pypi.python.org/pypi/pylint
+
+If you happen to find any issue reported by these programs, we would welcome
+you to fix them, as they are usually very easy to fix and we would all
+benefit from a better code base.