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
interesting - in this example, why not literal notation:

```jsig
foo : {id: String, version: String}
```

One thing that my above version doesn't include is the notion of type closure for the remaining object properties. If `Object<TKey, TValue>` means that every property of the object has a key of type `TKey` and a value of type `TValue` then that gets us that additional level of constraint expressivity.

> However using this would require defining what the semantics of & are.
cross referencing this to https://github.com/jden/jsig/issues/14, although from prior conversations I think we have a good idea of the semantics of & and |