Update new contributing guide changelog ref [skip travis]
@@ -31,7 +31,7 @@ to help you make a contribution.
* describe why you are proposing the changes you are proposing;
* try to not rush changes (the definition of rush depends on how big your
changes are);
-* Enter your changes into the [change log](https://github.com/jupyter/repo2docker/blob/master/CHANGES.rst);
+* Enter your changes into the [changelog](https://repo2docker.readthedocs.io/en/latest/changelog.html) in `docs/source/changelog.rst`;
* someone else has to merge your PR;
* new code needs to come with a test;
* apply [PEP8](https://www.python.org/dev/peps/pep-0008/) as much
