Add note to networking docs about iOS ATS exceptions

Summary:
JoelMarcey requested that I make this PR based on [my comment](https://github.com/facebook/react-native/commit/7ac931ee9b5e06ef1935d18e5b6a1e282db16c56#commitcomment-18270206).

I have signed the CLA.
However I was unable to verify that the site builds with this change. It looks okay in my markdown editor, but when following the contributor docs and running `npm install && npm start`, I get the following error when hitting node in my browser:

```
error in renderAPI for ../node_modules/react/lib/NativeMethodsMixin.js
Error: ENOENT: no such file or directory, open '../node_modules/react/lib/NativeMethodsMixin.js'
    at Error (native)
    at Object.fs.openSync (fs.js:634:18)
    at Object.fs.readFileSync (fs.js:502:33)
    at renderAPI (/Users/eliot/Dev/react/react-native/website/server/extractDocs.js:439:28)
    at concat.apis.map (/Users/eliot/Dev/react/react-native/website/server/extractDocs.js:601:14)
    at Array.map (native)
    at module.exports (/Users/eliot/Dev/react/react-native/websit
Closes https://github.com/facebook/react-native/pull/9107

Differential Revision: D3664102

Pulled By: JoelMarcey

fbshipit-source-id: 845917351ba9d3d2f5351a8f926757718c806025