What to Expect from Maintainers

Summary:
Adds a new maintainers guide, and updates the contributor's guide to be consistent with regards to this new guide.

Some additional style changes made in order to support the display of bot commands. Changed the wording for the "Edit this page on GitHub" link.

Finally, the contributor's guide is now synced to `CONTRIBUTING.md` on the repo.

```
cd website && npm start
```

Verify that `CONTRIBUTING.md` is updated whenever the website is regenerated.

Verify everything rendered correctly. Expand the details below to see screenshots.

<details>

![screencapture-localhost-8079-react-native-docs-contributing-html-1501016495792](https://user-images.githubusercontent.com/165856/28593706-33d1e03c-7142-11e7-9878-04ead7561abc.png)

![screencapture-localhost-8079-react-native-docs-maintainers-html-1501016508744](https://user-images.githubusercontent.com/165856/28593719-3812d7fa-7142-11e7-9db2-f9599057d726.png)
</details>
Closes https://github.com/facebook/react-native/pull/15202

Differential Revision: D5494246

Pulled By: hramos

fbshipit-source-id: e28d5624d1e4795e212f10e8d5713d91a0eae15f