Contributing guidelines

[foolip]

Someone asked me about guidelines for adding features, and I noticed that https://github.com/web-platform-dx/web-features/blob/main/docs/CONTRIBUTING.md doesn't say anything about this.

We should at least point to https://github.com/web-platform-dx/web-features/tree/main/docs#feature-guidelines and say how to pick compat_features and generate a .dist.yml file.

cc @ddbeck

[foolip]

@ddbeck provided an outline of documentation in #1159 (comment) and in today's meeting @captainbrosset volunteered to write a first draft of contributing docs, so assigning to him. Thanks @captainbrosset!