Added Read The Docs badge

Closes #109

README.md

@@ -2,6 +2,7 @@
 
 [![PyPI](https://img.shields.io/pypi/v/datasette.svg)](https://pypi.python.org/pypi/datasette)
 [![Travis CI](https://travis-ci.org/simonw/datasette.svg?branch=master)](https://travis-ci.org/simonw/datasette)
+[![Documentation Status](https://readthedocs.org/projects/datasette/badge/?version=latest)](http://datasette.readthedocs.io/en/latest/?badge=latest)
 
 *An instant JSON API for your SQLite databases*