Update README

This is for #46 and it's just a start. Opening it early to get feedback.

The general goal of this README is to easily explain why somebody would like to use ROM and how it differs from other ORMs and how you can get started quickly. It should also clearly state it's an experimental piece of software.

## TODO

- [ ] Come up with a brief and straight to the point introduction explaining core values and differences
- [ ] Have a nice CREDITS section with a paragraph about project's history (comes back to DM1, DM2 and so on
- [ ] Synopsis could be shorter (probably?)
- [ ] Consider splitting things up into small sections explaining key differences and features (with code samples?)

/cc @elskwid @jdickey