Add CONTRIBUTING document

[nastevens]

Contents can be modeled after rust-lang's document: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md

In addition to the content in that document, the following copyright should be applied to all new files:

// Copyright (c) 2018 The predicates-rs Project Developers.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.