Include default callback URL (OAuth)

What does this MR do?

Adds information on the OAuth callback URL

Are there points in the code the reviewer needs to double check?

None

Why was this MR needed?

This information was missing.

What are the relevant issue numbers?

http://www.twitter.com/designingcode/status/753511397124866049

Does this MR meet the acceptance criteria?

• [X ] Documentation created/updated