Add a CONTRIBUTING based on UCL-ARC/python-tooling.

Author: samcunliffe

CONTRIBUTING.md

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+# Contributing Guide
+
+This repository is a collection of best practices and coding standards. It has been made by [research
+software engineers] at [UCL's Centre for Advanced Research Computing][UCL ARC].
+Some of it is from our internal documentation and recommendations pulled out and made public.
+The content is stored in [GitHub flavoured markdown], and is rendered as a live [webpage].
+
+Any changes should be reviewed by a member or associate member of ARC.
+If the additions are taken from ARC's internal documentation, **please add the original author** as a reviewer to check that they're OK with their content being made public a CC-by-4.0.
+If something seems like it's broken, please go ahead and [open an issue]!
+
+> [!NOTE]
+> We're open for contributions from anyone but it's the UCL ARC recommended best-practices, so ulitmately it belongs to the software teams at ARC and we may not adopt suggestions from outside (sorry!).
+
+## Development workflow
+
+To contribute a change, please:
+
+1. Create a new branch (or fork if you're not in [@UCL-ARC/collaborations]).
+2. Modify an existing page, or create a new one.
+    a. Run [pre-commit] which will lint your changes.
+3. Open a _pull request_ (PR) with changes.
+4. Ask someone who did not contribute to the PR from [@UCL-ARC/collaborations]
+   to review it.
+5. If approved with no comments, then the last approving reviewer should merge the PR.
+6. If changes are requested, the PR author should address the comments, and then
+   ask for review again.
+
+<!-- links here -->
+
+<!-- prettier-ignore-start -->
+
+[GitHub flavoured markdown]: https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax
+[webpage]: http://github-pages.arc.ucl.ac.uk/coding-standards/
+[UCL ARC]: https://ucl.ac.uk/arc
+[open an issue]: https://github.com/UCL-ARC/coding-standards/issues/new/choose
+[Research software engineers]: https://society-rse.org/about/history
+[pre-commit]: https://pre-commit.com
+[@UCL-ARC/collaborations]: https://github.com/orgs/UCL-ARC/teams/collaborations
+
+<!-- prettier-ignore-end -->