Issue #7: Add badge for readthedocs status.

README.rst

@@ -14,6 +14,9 @@ scikit-surgeryvideolag
     :target: https://weisslab.cs.ucl.ac.uk/WEISS/SoftwareRepositories/scikit-surgeryvideolag/commits/master
     :alt: Test coverage
 
+.. image:: https://readthedocs.org/projects/scikit-surgeryvideolag/badge/?version=latest
+    :target: http://scikit-surgeryvideolag.readthedocs.io/en/latest/?badge=latest
+    :alt: Documentation Status
 
 scikit-surgeryvideolag is a python app to display a millisecond timer along with an OpenCV video feed.
 You can point your video camera back at the screen, take a screenshot,