Add CONTRIBUTING.md

[lukehoffmann]

GitHub prefers to have an onboarding document at the top level of the repo called CONTRIBUTING.md. One benefit is that you get a "Contributing" tab in the readme area on the project home page, i.e.
https://github.com/lukehoffmann/PyEarthTools/tree/contributing?tab=contributing-ov-file#

See GitHub documentation Setting guidelines for repository contributors

This adds a simple document, which will initially be mostly links to other documentation

[lukehoffmann]

I referred to contributing.md in the IMPROVER repo, which mostly just links to more detailed pages in the full documentation for that project
https://github.com/metoppv/improver/blob/master/CONTRIBUTING.md

I've just summarised part of that pinned issue and part of the devguide.md, while linking to both of those for further details.

[lukehoffmann]

According to the GitHub docs the name of this file is not case sensitive (I think it used to be but it has been relaxed)

Decide whether to store your contributing guidelines in your repository's root, docs, or .github directory. Then, in the filename field, type the name and extension for the file. Contributing guidelines filenames are not case sensitive.

so I've changed it to docs/contributing.md

[tennlee]

This look great, thanks.

[coveralls]

Pull Request Test Coverage Report for Build 19283317083

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.321%

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Totals
Change from base Build 19281533898:	0.0%
Covered Lines:	9584
Relevant Lines:	15214

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💛 - Coveralls