Provide JSDoc

[emmenko]

Just for keeping track of this.

[gaearon]

I'm on board but let's wait for some conclusion on #87.
Should we annotate types both in Flow and in JSDoc? What's the usual approach here?

[emmenko]

I haven't used Flow yet, so I can't really say what's the best approach. I think it also depends if you just want JSDoc in the source code or use it to actually generate a documentation website or something like it.

[gaearon]

This was done via #300.