Improve and split `CONTRIBUTING.md`

[llucax]

What's needed?

The CONTRIBUTING.md file is mixing a couple of different topics:

• Setting up a development environment
• Build instructions
• How to install from sources
• Instructions for maintainers (how to release, how cross-arch testing works)

And it is also missing some important information:

• What's expected in PRs
• How people wanting to contribute should communicate with us

The later is probably the most important information for the CONTRIBUTING.md file, as it is shown to users when trying to contribute by opening a PR or an issue.

Proposed solution

Split the CONTRIBUTING.md file into:

• DEVELOPMENT.md: Instructions on setting up a development environment, how to run the tests and build instructions. Anything related to development that is useful even if you are not planning to contribute back.

• maintainers-guide.md (in docs/): Instructions for maintainers (how to release, how cross-arch testing works in the CI, etc.)

• CONTRIBUTING.md: Instructions on how to contribute back. For example:

  • How to contact us
  • How should you write commit messages
  • How to write release notes
  • How to write tests and how much tests we expect in PRs
  • What to do before even opening a PR (like create an issue or discussion to make sure the PR has chances to get accepted)
  • Explain how to sign commits, and to add a sign-off-by, what DCO is and why we require all of this
  • Check-list for PRs, like running nox, writing tests, writing release notes
  • Some more ideas in https://github.com/danielgtaylor/python-betterproto/blob/master/.github/CONTRIBUTING.md

We should probably also add a documentation top-level section Developers, where we could group these 3 documents into different sections. Like:

Additional context

• Improve documentation on how to install from sources #246