Document how to setup the "authorization_code" grant

[spideyfusion]

[v-m-i]

@okazy
Hi, I have created documentation for authorization_code grant, it is still work in progress and I would appreciate your feedback on it. #177

[okazy]

@v-m-i
Thank you for your mention.
I checked and made some comments.

[pszalko]

This PR is just in time! I just spend an hour debugging why $authRequest->setAuthorizationApproved() is set to false and code for authorization in listener is never actually called.

BIG thanks!

[thomas-hiron]

This definitely needs to be merged! I spent too much time on how to approve the request before finding this PR.