Update readme and other non-technical docs

[ehmatthes]

• [x] Update README.
  • project description
  • contributing
• [x] Update vision.md
  • necessary?
  • remove it for now; revive it when I have time to write a clearer vision document. Roadmap is more important at this point.
• [x] Update glossary.md
  • necessary? brief, but link to external docs?
  • Revise this as I clean the code and transition to gsp taxonomy in code.
• [x] Update openlearningtools.md
  • necessary, or redundant with vision?
  • explain origin of openlearningtools, and where it might go at this point?
  • get rid of this, revive it in whatever newer form it takes.

[ehmatthes]

Some work left to do, but in conjunction with refactoring.