Contributing guide

Hello, thanks for the useful yaml library.

I propose adding a CONTRIBUTING.md guide for this project, because this project still appears very popular:

• it still shows up as the first result of pkg.go.dev for yaml, with the biggest "Imported by" value of 24k: https://pkg.go.dev/search?q=yaml&m=
• it is still listed on the "Popular Go Packages" section of libraries.io: https://libraries.io/search?order=desc&platforms=Go&sort=rank

However:

• the project's last commit was almost 3 years ago
• there are currently 128 open PRs and 294 open Issues.
• @niemeyer has mentioned that the project needs help here: https://github.com/go-yaml/yaml/issues/788

I think a CONTRIBUTING.md could mention things like:

• any style guidelines that this project follows
• how to get in touch with the maintainer(s)
• some rules on creating Issues/PRs

Some good examples:

• simple: https://github.com/gin-gonic/gin/blob/master/CONTRIBUTING.md
• more detailed: https://github.com/gohugoio/hugo/blob/master/CONTRIBUTING.md
• very detailed: https://github.com/kubernetes/community/tree/master/contributors/guide#getting-started
• external docs example: https://github.com/pandas-dev/pandas?tab=readme-ov-file#contributing-to-pandas

I think having this file would be a good sign that "help is wanted/needed".

If this is a good idea, I can volunteer to give it a shot.