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
This is indeed possible. I've used the following pattern before:

```js
foo : Object<"id", String> & Object<"version", String> & Object<String, T>
```

However using this would require defining what the semantics of `&` are.