write docs for derive and contract crates

[debris]

[fubuloubu]

This could really help me out as I've been exploring how to use this functionality for https://github.com/ethpm/ethpm-rs

So far I figured out this:

#[derive(EthabiContract)]
#[ethabi_contract_options(path = "MyContract.abi")] // Note: relative to build location
struct MyContract;

Haven't figured out how I am actually supposed to use the struct lol

[BlinkyStitt]

This has been helpful for me: https://github.com/openethereum/ethabi/blob/master/tests/src/lib.rs