Add some notes about using TypeScript and ES6

[jmsche]

This small PR to add details about writing TS controllers (at least, to avoid errors by just writing a simple one).

Thanks @weaverryan for finding the issue I had in #59 :)

I added your explanation almost word for word, I hope it's okay :)

[weaverryan]

Thank you @jmsche - this is exactly what I had in mind!