Update README to include system prerequisites and usage

[fordprior]

Some basic usage and prerequisites would help people quickly understand the high-level scope and context of the project. It also might help people get started.

For example, in a Prerequisites section:

We recommend a, lots of people use b, and we don't recommend c. Also, this repo assumes strong understanding of x, y, and z.

And, for example, a Usage section:

To get started: -clone x repo, set up y in AWS, run z commands"

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/156650821

The labels on this github issue will be updated when the story is started.