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
That's correct in ES5, in ES6 keys can be Strings or Symbols.

I've left the first argument in there so that you can use a named alias to express intent.

For example `version: Object<SemverVersion, T>` where `SemverVersion` is a named alias of `String`. This is a common technique in typed languages to use the type expression to represent human readable intent rather then machine readable intent.