No obvious contribution guidelines

[pmj]

First of all, thanks for maintaining these low-level macOS framework bindings and wrappers. I wouldn't have been able to start using Rust for some small work projects (which I'm hoping to use as a springboard for bigger things) if I'd had to put in the extra work of creating my own bindings.

I came across a minor omission in the core-graphics crate, which I've rectified locally and put into a pull request (#452) - but I've not found any notes in the readme, GitHub wiki, or elsewhere about what exactly the protocol is for submitting contributions. Even if it's just a one-line "If you wish to start contributing or even make a one-off change, simply submit a pull request with the code or documentation change and we'll go from there," then that would be helpful for people who find themselves in the position I found myself in just now.

[jrmuizel]

This crate is a bit under maintained so we don't really have anything like that. That being said, I'd say your one-line contribution guideline is a good place to start and would accept a PR adding it :)