Update readme

Author: agjohnson

README.rst

@@ -5,7 +5,13 @@ Read the Docs - Docker Images
 These are the Docker image definitions used by the Read the Docs build
 environments to encapsulate the build process.
 
-You can find these images on Docker Hub, on the `readthedocs/build`_ repository.
+You can find these images on Docker Hub, on the `readthedocs/build`_
+repositoryr:
+
+* `readthedocs/build:1.0` - Ubuntu 14.04 based image
+* `readthedocs/build:1.0-dotnet` - Ubuntu 14.04 based image, plus .NET
+* `readthedocs/build:2.0` - Ubuntu 16.04 based image
+* `readthedocs/build:latest` - Latest development image
 
 .. _readthedocs/build: https://hub.docker.com/r/readthedocs/build/