Add documentation to the readme

[fredemmott]

This is the only real blocker to release. Should cover installation and basic usage - both from perspective of the CLI tool, and for writing test cases.

[wlin53]

Does CONTRIBUTING.md need to be changed?

[fredemmott]

Yep - the current one is rather unique to the HSL.

https://github.com/hhvm/hh-apidoc/blob/master/CONTRIBUTING.md is probably a good start to use as a template - changes would be:

• remove the API reference section (we can re-add it once the repo is public)
• remove references to HH-APIDoc and HH-CLILib (whoops, HH-CLILib shouldn't be there in the hh-apidoc repo)
• update the 'core components' section

[wlin53]

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