Adding README for acceptance test directory

Adding a landing page doc file to help new users with running acceptance tests. Right now there are a lot of environment variables that are non-obvious to set up.
One note here: rather than setting these by hand, you can actually download and source an RC file from Horizon, although it's a little hard to find:

![screen shot 2014-09-09 at 10 46 45 am](https://cloud.githubusercontent.com/assets/17565/4203557/4722da72-3830-11e4-930a-2c76ec5ee25f.png)