Extended documentation

[analyticalmonk]

The documentation (README.md) doesn't give out a lot.
To figure out something even as simple as validating the secret token, I'd to go through the source code. That's not ideal.
I'll be using the framework in a couple of projects. I can help with the documentation and the repo admins can review it if that's acceptable.

[alex-ball]

It would have saved me a lot of debugging time if I had realised that you can't validate the signature from GitHub if the ping is sent as application/x-www-form-urlencoded (since request.data is then an empty binary string).

Some documentation of using the secret keyword argument would be greatly appreciated. Again, I'm happy to contribute this if the admins would like.