Clarify what the callback url should be for github backend

[myers]

In order to get my app to work with github I need to set the callback url to:

http:///complete/github/

Currently the docs only say:

Register a new application at GitHub Developers, set your site domain as the callback URL or it might cause some troubles when associating accounts,

I think it should read

Register a new application at GitHub Developers, setting the callback URL to "http:///complete/github/".

If this seems like the right idea I'll be happy to do the work.

[omab]

@myers, go ahead, all contributions are welcome.