License, Contributing and Readme rejig

- Added Middleman's standard `CONTRIBUTING.md` (with a modifcation for there being no need to write / run tests).
- Presumptuously added an MIT `LICENSE.md` ( I assume thats ok?).
- Rejigged the `README.md` to be a little more consistant with Middleman's other `README.md`s
- Removed `i18n` warning as it should not be necessary going forward.

Thanks again for this example living in its new home.