Shorten installation section.

People know how bundler works by now.
Don't need strong parameters

Since operations automatically filter the given parameters into the
(white)list of attributes they specify, we don't always need the
controller to do this whitelisting again. This cleans up the example
code a bit.
Remove section on callbacks

Since we decided to postpone support for them, we shouldn't keep it in
the README.
Add example showing how to use the context
Rewrite section on authorization to show examples