Pdpinch/documentation updates

All the docs, including installation and course author documentation, are in the ReadMe now. 
Technically, I agree that xblocks like this should be included as optional apps, but conceptually it seems that there needs to be some kind of dependency checking between courseware and xblocks (i.e. if xblocks are missing that an imported course needs, it should be handled gracefully as well as notify ops as soon as it's possible for it to be known - before the platform even starts taking requests).
That would be ideal, but if such a mechanism doesn't exist yet, wouldn't it be the safer route to assume that if someone is adding any xblock to their production platform, they're promising support for it to course developers, making it not really "optional"? It seems to me that, assuming they're promising support, it's better to have it fail upfront than to wait until the platform throws a 500 at a user, which will ultimately end in an app restart anyways.

I think the risk of sga going missing is low, so it's not really a big deal and I might just be over-complicating it, or hitting up against something I don't know.