Start writing the CONFIGURING.md doc.

todo:

- expand on some of other .env settings
- add info to the README that points to the doc
Restructure CONFIGURING.md and some info to README
Add back Twitter config to .env.example.

Twitter has not yet been removed from this, but will be when oc-id gets merged
in.
Merge pull request #238 from opscode/configuration_docs

Writing the CONFIGURING.md doc.