README - add link to issue tracker (#66)

As discussed in #64

Author: adamchainz

README.rst

@@ -61,6 +61,8 @@ Share your feedback and ideas: `subscribe to the mailing-list
 Contributing
 ------------
 
+Issues are tracked on `Launchpad <https://bugs.launchpad.net/pyflakes>`_.
+
 Patches may be submitted via a `GitHub pull request`_ or via the mailing list
 if you prefer. If you are comfortable doing so, please `rebase your changes`_
 so they may be applied to master with a fast-forward merge, and each commit is