Install.rst: Add link for the Docker documentation

Adds the link for the coala Docker documentation giving
users more information on how to use coala with Docker.

Closes coala#588

Author: rohanrajpal

Users/Install.rst

@@ -183,6 +183,7 @@ virtualenv with:
 
 Using coala With Docker
 =======================
+Don't know what Docker is? Have a look at `What is Docker? <https://docs.coala.io/en/latest/Users/Docker_Image.html>`_
 
 Use this installation method if you *just simply want to use coala*.