Link to contributor documentation from GitHub README

[trey-stafford]

Migrated from Jira:

e.g. A link to "How to contribute a change to a layer style" would make it easier for potential contributors to evaluate the difficulty of a task. Inspired by feedback in user research doc:

But I think there's agroup of us who are a little more technical who might contribute via like a pull requestor something, but that level suddenly gets really technical, like there's, you know,custom tools I have to install and workflows if I actually want to test that what I did iscorrect and I feel like there should be a middle ground where it might be easier tocontribute. Even just contributing layer style files, so you have the layers, and just if thisfile is there, it will look a certain way and I guess I could just email those.

[trey-stafford]

Resolved by #711 : we link tot he contributor docs from the README. These docs have some more entry-level (e.g., contributing styles) how-tos that address the comment above.