Add more info into README

Author: shihanng

README.md

@@ -1,3 +1,6 @@
 # iruby-docker
 
-iruby-docker is a community maintained Jupyter Docker Stack image
+iruby-docker is a community maintained Jupyter Docker Stack image
+with [iruby](https://github.com/SciRuby/iruby) installed.
+This image is built based on the [Jupyter Docker Stacks's cookiecutter],
+the image is available on [Docker Hub](https://hub.docker.com/r/shihanng/iruby-docker).