Add instructions for stripping type annotations

This gives people who use flow type annotations an immediate solution, though it would be nice to either strip types out automatically, or add a stripTypes option.

I looked to see if there was a way to get the stripTypes functionality pulled out into a module, but it seems pretty enmeshed in the [react source code](https://github.com/facebook/react/blob/e8e79472aabcbcaa70ad8cd901722cad2dbbd709/main.js#L40-L46).