Auto merge of #489 - jhwgh1968:crate-doc, r=jdm
Add crate docstring and two code examples
In the hopes of embedding SpiderMonkey into a non-Servo Rust project, I spent some time digging into these bindings. Unfortunately, it was very easy to get lost.
This PR is my attempt to make it easier for the next person who attempts my exercise by adding a few sign posts I would have found helpful, and translating one of the basic examples from the SpiderMonkey User Guide on MDN to Rust. Documenting the entire crate (and where it dips directly into ``mozjs_sys``) is a task too big for any one person, I think.
EDIT: removed WIP comment
