Add instructions for stripping type annotations

This gives people who use flow type annotations a 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).
Update README.md

Updating README as suggested.
switch from exec to execFile
remove redundant code
log a warn message if .flowconfig is missing
x# This is a combination of 6 commits.
- Use flow status instead of check

- killFlow option for those that don’t want to leave the server running

- Updating docs

- Adding [libs] to .flowconfig

- Make JSHint pass

- Undo [libs] addition

- Remove npm-debug log
- Update .flowconfig, remove declarations test