Add RTD badge

Author: mwcraig

README.md

@@ -1,5 +1,7 @@
-# astro-image-display-api
+# AIDA, the Astronomical Image Display API
+
+!
+[![Documentation Status](https://readthedocs.org/projects/astro-image-display-api/badge/?version=latest)](https://astro-image-display-api.readthedocs.io/en/latest/?badge=latest)
 
-Astronomical image display API definition for cross-backend interoperability
 
 Also see: https://github.com/astropy/astrowidgets