Create a developer guide

There are a number way and places to extend the PerfKitBenchmarker and we are at the point of needed a developer guide on how to;
- Add a new benchmark and the defined APIs for doing so
- How to add a new package
- How to support a new image type
- Etc.

We have a number of internal contract we've done well to keep to and some we are still cleaning up like the flags vs config objects.

A developer guide will go a long way to driving consistency and reuse inside the framework.