Updated README based on the notes from our chat
Updated README based on the notes from our chat

Still needs (from you guys):
- main dependency to the credentials directory and credential files, or do a small unit test to show how to use it
- under "Usage," an example with two different tokens with two different scopes
- a test case with the “test credentials” in JSON files that uses the normal setup, and mock the OAuth provider with some mock REST service
- an example showing how we use it
- more documentation of the config options
zalando-stups/tokens