Update *chaining assertions* section

chaining assertion is a good tool for should.js, however its document is unclear.

We should tell the user that:

1. and, with, of, have, be, a. They all dont have any effect but for readability.

2. `property` and `ownProperty` return the property value but not their hosted object. While other assertion just return self.
I dont realize that before, and I think `and` getter means parallel relationship while `with` means progressive relationship. Today I read source and know my understand is false. 

So, I make it clear in README.