OAuth Application Section

Hey Rich,

Product Management wanted official documentation on the OAuth application before it could be released to Liferay Marketplace. I've included in this pull request a section for the OAuth app. My next step is documenting OAuth for the Dev Guide, which will piggyback off of the content I wrote for the User Guide.

I wasn't sure if you were aware of the timeline for the OAuth app's release to Marketplace, but we'll need to wait to publish this section until the app is publicly available.

Thanks!