2.5 Completion Shell

Created a simple page for the new completion shell that comes with 2.5. It's not perfect so if you spot any issues please comment and I'll correct them.

P.S. I'm not much of a documentation writer but I do this to get better at it and to fulfill [a way over due promise I made a while ago](https://github.com/cakephp/cakephp/pull/1644#issue-19548652).