document generic objects

Added an informal section to the README showing concrete examples for how to express generic objects using jsig.

This fixes #40 

cc @Matt-Esch @jden
So I could also use the following syntax

`foo : { id: String, version: String, ...: T }` which would also solve the problem, this requires the non-standard jsig extension of `...: T` that I talked about in another issue.

The original one doesn't require any new syntax but it is ambigious from a machine level, not a human level.