Document hooks in readme

[jordaneb]

Hooks are currently undocumented but can be very handy; particularly failure hooks. Here's some basic documentation for them so that it's easier to discover them!

[j4mie]

This is great but there's a really important detail that we should add - the creation hook happens in the save of the Job model, so will run in the process that creates the job (ie usually the web server process). The failure hook runs in the worker process (if the job fails).