added docs status badge to readme

Author: dantownsend

README.md

@@ -4,6 +4,8 @@
 
 [![Coverage Status](https://coveralls.io/repos/github/piccolo-orm/piccolo/badge.svg)](https://coveralls.io/github/piccolo-orm/piccolo)
 
+[![Documentation Status](https://readthedocs.org/projects/piccolo-orm/badge/?version=latest)](https://piccolo-orm.readthedocs.io/en/latest/?badge=latest)
+
 A fast, user friendly ORM and query builder which supports asyncio.
 
 Not production ready yet, but progressing quickly.