Fix missing URL in README

Author: shihanng

README.md

@@ -7,5 +7,5 @@
 
 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],
+This image is built based on the [Jupyter Docker Stacks's cookiecutter](https://jupyter-docker-stacks.readthedocs.io/en/latest/contributing/stacks.html),
 the image is available on [Docker Hub](https://hub.docker.com/r/shihanng/iruby-docker).