message: Add CircleCI badge.
diff: RTK: The Reconstruction ToolKit
  +.. image:: https://circleci.com/gh/SimonRit/RTK.svg?style=shield
  + :target: https://circleci.com/gh/SimonRit/RTK
  +
  .. image:: https://travis-ci.org/SimonRit/RTK.svg?branch=master
  :target: https://travis-ci.org/SimonRit/RTK/
  