Adding my first draft of the OAuth2 Adapter code  to the readme

First go at this, this is using League OAuth 2.0 client 0.3.0 and dev-master of Aura.Auth - I've written the code and tested it and it works. Just let me know if anything reads weird or any other changes you have, also if I need to make this PR against master (which I just now considered, I can do that too).