Add automated documentation generation

[thibaudcolas]

At the moment, the project's documentation is limited to what is in the README. It would be nice if there was more complete documentation generated automatically: a full API reference, with links back to the source code, based on the Flow types (or JSDoc).

Ideally this would be generating Markdown so it can be versioned with the code in Git/GitHub.

Tools to consider & useful info:

• https://github.com/documentationjs/documentation
• https://github.com/esdoc/esdoc
• https://survivejs.com/maintenance/documentation/readme/
• https://survivejs.com/maintenance/documentation/api/

[thibaudcolas]

See thibaudcolas/draftjs-filters#19, thibaudcolas/draftjs-filters#26 for a solution.