Various minor updates to the main Readme file

Several small updates to the main Readme file:

  - Addition of a documentation index
  - Clarification of headers (to clarify what is for OAuth in general and what is for this library)
  - Addition of unlinked files (scope/response)
  - Emphasis on step-by-step walkthrough. This is all people want when first visiting! It used to stand out really well in the previous version (pre-cookbook); this should improve it.