Add a CONTRIBUTING.md (#30)

based on UCL-ARC/python-tooling.

@paddyroddy strong opinion about the merging strategy?

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Milan Malfait <38256462+milanmlft@users.noreply.github.com>
Co-authored-by: Patrick J. Roddy <patrickjamesroddy@gmail.com>

CONTRIBUTING.md

@@ -0,0 +1,37 @@
+# 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 under a CC-by-4.0 license. If something seems like it's broken, please go ahead and [open an issue]!
+
+<!-- prettier-ignore -->
+> [!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 -->
+
+[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