Adds version details in README file

Describe elixir and phoenix versions used in the dockerfile

Author: ronualdo

README.md

@@ -4,4 +4,5 @@ Docker image for use on API projects using phoenix framework. This image provide
 phoenix framework and basic dependencies. Since it was not intended to support any
 client side work, it does not install Node, Brunch, PostgreSQL or any of their dependencies.
 
-This image is build on top of the official elixir alpine based image.
+This image is build on top of the official elixir alpine based image. It provides elixir
+1.6.5 and phoenix 1.3.2.