Add readme

@pcostell I took a stab at porting to `git` and detailed a bit of the process in the `README`.

Do you guys have any interest in
1. Moving to GitHub
1. Dropping the "experiment" from the repo name.
1. Integrating with Travis.

My main reason for doing this was to try to get `ndb`-like features into `gcloud-python`. (Really just `Property` and `Model` and relevant subclasses for starters.)

/cc @silvolu