Custom readme for react-tools

Of course this means I can no longer just npm publish from the project
root, but I haven't been anyway.

I've been meaning to do this for a while and right before a release seems like as good a time as any. I actually want to have a completely separate `package.json`, but didn't want to overreach here.

If anybody has suggestions for better content, *please* share. I wrote this quickly intending for it to get some love before actually releasing.