Improve user friendliness of CONTRIBUTING.md

[Sophist-UK]

Summary

• This is a…
  • Bug fix
  • Feature addition
  • Refactoring
  • Minor / simple change (like a typo)
  • Other

Add some more general preamble to CONTRIBUTING.md:

• to be a more friendly introduction to contributing for newcomers; and
• to cover ways to contribute other than Picard code.

[phw]

@Sophist-UK Thanks for your contribution. Could you rebase your branch against master? I had just some Markdown linting changes merged with #2813 . Also Codacy is now removed.

Github actions is currently acting up, though, expect tests to fail. But we can rerun them once Github is stable again.

[Sophist-UK]

@phw Phillip - I think I have rebased it. Let me know if there is still an issue.

And given that this is solely a markdown file change, I didn't bother checking that the tests still worked.

[phw]

Thanks again. Looks good to me, good to have a broader introduction here. I have only some minor content related comments.

Also our Markdown linting is complaining. But it is only two issues:

1. Use dashes for lists instead of asterisk for consistency
2. Get rid of trailing whitespace in several lines

[Sophist-UK]

@phw @zas This PR only changes a single *.MD file which is NOT a code change and is not included in a package, yet it runs CodeQL, package release checks, various non-MD code linting, and all the unit tests, none of which have any relevance to this PR.

So as an aside, may I suggest that we reduce our automation carbon footprint by adding filters to each Github workflow so that it only runs when either the workflow yaml file itself has changed or the relevant types of file have changed.

[phw]

@Sophist-UK Yes, we should limit the unit tests. I'll look into this. We already have the packaging restricted.

The linting is probably more trouble to check whether something has changed then to just run it, since we move all those checks into a single workflow file, which makes maintenance much easier for those small checks.

The PR here looks good. I'd like to have this reviewed by @zas before merging, though. He did the last big update to this file.

[zas]

LGTM