Restructure docs

[judgej]

The README is a little confusing now, since it was originally supporting just Partner apps, and now has Private apps added to it.

Essentially the difference between Partner and Private apps here is teh way in which the OAuth token is:

a) Initially set up (it needs user authorisation for the Partner app, but is fixed for the Private app).
b) Renewals are handled (the Private app has no token renewals, while Partner apps support callbacks for storing token renewals when they happen).

So restructure so make it clearer what setup is specific to an app type, and what setup is common for all app types.