Fix link in README.
Use format that's detected by setup.py's conversion functionality so
that the link works in PyPI.
@@ -134,7 +134,7 @@ Contributing
------------
Interested to contribute to Robot Framework? Great! In that case it is a good
-start by looking at the `Contribution guidelines <CONTRIBUTING.rst>`_. If you
+start by looking at the `<CONTRIBUTING.rst>`__. If you
do not already have an issue you would like to work on, you can check
issues with `good new issue`__ and `help wanted`__ labels.
