Mention body parsing in README

[yortus]

This lib assumes req.body has already been parsed by the time the decorated router sees it. The README should be more explicit about this. A common gotcha is seeing validation errors on perfectly valid requests, where the problem is actually caused by a lack of body parsing middleware, so req.body isn't defined.

Maybe the error message when req.body is undefined could even provide a hint about this, since it's a likely cause.