docs: 📝 add CONTRIBUTING file

CONTRIBUTING.md

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+# Contributing
+
+## Issues and bugs :bug:
+
+The easiest way to contribute is to report issues or bugs that you might
+find while using `template-data-package`. You can do this by creating a
+[new](https://github.com/seedcase-project/template-data-package/issues/new/choose)
+issue on our GitHub repository.
+
+## Adding or modifying content :pencil2:
+
+If you would like to contribute content, please check out our
+[guidebook](https://guidebook.seedcase-project.org/) for more specific
+details on how we work and develop. It is a regularly evolving document,
+so is at various states of completion.
+
+To contribute to `template-data-package`, you first need to install
+[uv](https://docs.astral.sh/uv/) and
+[justfile](https://just.systems/man/en/packages.html). We use uv and
+justfile to manage our project, such as to run checks and test the
+template. Both the uv and justfile websites have a more detailed guide
+on using uv, but below are some simple instructions to get you started.
+
+To install uv and justfile, run:
+
+``` bash
+pipx install uv rust-just
+```
+
+We keep all our development workflows in the `justfile`, so you can
+explore it to see what commands are available. To see a list of commands
+available, run:
+
+``` bash
+just
+```
+
+As you contribute, make sure your changes will pass our tests by opening
+a terminal so that the working directory is the root of this project
+(`template-data-package/`) and running:
+
+``` bash
+just run-all
+```