Pdpinch/documentation updates

All the docs, including installation and course author documentation, are in the ReadMe now. 
So I think the first part of my comment isn't really an issue, but I'm still concerned about OPTIONAL_APPS. Is the use of OPTIONAL_APPS okay here @carsongee? 

"It seems to me that, if someone intentionally adds edx-sga to their platform, they'll want the service to not run if it disappears as it's absence might adversely affect live courseware."