Documentation for Ad Unit V1 API

A simple document to start with for Pre-Packaged Marketing Ad Unit.

Attaching screenshot of the documentation.
![pre-packaged marketing ad units - offerengine api](https://f.cloud.github.com/assets/3762885/1409970/19441f60-3d9c-11e3-8c5c-27f9246d0c09.png)