Pdpinch/documentation updates

All the docs, including installation and course author documentation, are in the ReadMe now. 
Is it the norm to encourage users to modify common.py in production? The best practice is to always keep all instance-specific config away from the repository and modify server-vars.yml, no?

I understand that OPTIONAL_APPS is processed in common.py and thus there is no way to process OPTIONAL_APPS using the JSON config right now, but why would that be desirable for xblock installations anyways? 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 that might adversely affect live courseware.