Recreate changelog as markdown

github-activity, used for other JupyterHub release processes,
now outputs markdown. This PR moves the existing changelog
to use markdown, and provides a more complete list of
changes too.

Ref #1280

Author: yuvipanda

CHANGES.rst

@@ -1 +1 @@
-The ``repo2docker`` changelog has been moved. It can be viewed online at https://repo2docker.readthedocs.io/en/latest/changelog.html. In the ``repo2docker`` repository, the changelog is located at ``docs/source/changelog.rst``.
+The ``repo2docker`` changelog has been moved. It can be viewed online at https://repo2docker.readthedocs.io/en/latest/changelog.html. In the ``repo2docker`` repository, the changelog is located at ``docs/source/changelog.md``.