Documentation

# Introduction
It's time for ReactiveUI to grow up and have versioned documentation and better samples!

http://reactiveui.readthedocs.org is being built automatically from https://github.com/ghuntley/reactiveui/tree/readthedocs upon every checkin.

If you want to help out with this effort please comment on this issue and I'll add you as a contributor to my personal repository.

## Operations:
- [x] Use Readthedocs.org to automatically build ReactiveUI documentation as it supports versioned documentation via branches.
- [ ] CNAME created for [docs.reactiveui.net pointed towards readthedocs](http://read-the-docs.readthedocs.org/en/latest/alternate_domains.html).
- [ ] Determine what to use and how to automatically build class documentation.

## Documentation:
- [ ] Structure of documentation defined.
- [ ] Migrate existing docs/* across.
- [ ] Clean existing docs/* to drop all stale Rx4 and Rx5 references.

## Samples:
- [ ] Ideas?