Added react-tools and jsx requirements

Adding the following comments in case someone already has `jsx` installed & faces the issue I did.

Got a `jsx: command not found` when I first ran `./rebuild`. So I installed `jsx` & got `unknown option: --harmony`. That's a `react-tools` flag, which I didn't have installed and trying to install it via npm gave me the following error.
```
npm ERR! Refusing to delete: /usr/local/bin/jsx not in /usr/local/lib/node_modules/react-tools
File exists: /usr/local/bin/jsx
Move it away, and try again.
```

So I deleted that jsx file and the react-tools installation went smoothly and I was able to `./rebuild`