[Docs] Document module publishing

There are at least 3 ways to publish a typed module to npm:

1. Transforming types into comments with [babel-plugin-transform-flow-comments](https://www.npmjs.com/package/babel-plugin-transform-flow-comments)

2. For each `file.js` publish `file.js.flow`, [flow-copy-source](https://github.com/AgentME/flow-copy-source) automates this

3. Built-in `gen-flow-files` command to produce compact annotations file (seems kinda buggy currently)

I did not find this information in the docs, maybe we should document this? I can send a pull request.