Create developer documentation

[jonathanbossenger]

Create documentation for developers on how to register and use Abilities.

See Feature API docs for reference.

We can probably reuse what we need from he Feature API docs, but at a minimum we probably need:

1. Intro
2. Getting started
3. Registering Abilities
4. Using Abilities
5. REST API endpoints

The repo readme should also be updated to include a "quick start" example, as well as links to the rest of the docs.

[jonathanbossenger]

Following on from the discussion on immediate use vs core adoption:

do we want to take some preventative measures (prefixing or namespacing) to allow for immediate 3rd party adoption/use of the Abilities API without worrying about carrying tech-debt into core with us or breaking the early-ecosystem

@justlevine, I want to explore your concern here, so we're on the same page. Perhaps you could share an example of what you're talking about.

[gziolo]

@jonathanbossenger, should we reopen to add the section about REST API usage?

[jonathanbossenger]

@gziolo I think let's create new issues as we add new features/functionality that require doc updates. Also, it makes it easier for multiple contributors to take part.

[jonathanbossenger]

Added #28 to start.