Add FreeAgent dev docs

> [FreeAgent](https://freeagent.com/) - accounting software for freelancers and small businesses. The FreeAgent API provides secure programmatic access to the FreeAgent service, allowing you to build cool and useful tools and services on top. The API uses OAuth 2.0, supports XML and JSON responses and organises resources according to REST.

I've linked straight to our docs index page, rather than the introduction as that's probably more useful for returning devs. :smiley_cat: