Add a very basic contribution guide

CONTRIBUTING.md

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+# Contributing to Puya
+
+Contributions of all kinds are welcome. For new features, please open an issue to discuss first.
+
+## Workflow
+
+[Conventional commits](https://www.conventionalcommits.org/en/v1.0.0/#summary) are used.
+Not all commits require a convention, but in general a user-facing change should include at least
+one commit with either a `fix:` or `feat:` commit, so that is included in the release notes.
+Other conventions such as `docs:` or `test:` are not necessarily required but may be helpful to
+see when reviewing.
+
+## Local development tips
+
+`poetry` is used for virtual environment and dependency management.
+
+Dependencies for building docs are in a separate `docs` dependency group, this is only required 
+if you want to build the documentation locally to see the effects of any edits you made.
+
+`ruff` is used for linting and formatting, `mypy` is used for static analysis. `pre-commit` is
+configured, so you will want to either install the hooks with `pre-commit install` or you can run
+things manually via `pre-commit run --all-files`.
+
+Testing uses `pytest`. You'll need to have a local `algod` node running for most of the tests
+to run, see [algokit-cli](https://github.com/algorandfoundation/algokit-cli).
+
+Part of the tests include checking for no output changes to `examples/` or `test_cases/`. If there
+are deliberate changes, you should commit those for the tests to pass.
+
+Additionally, if there are any material changes to the output (ie the generated code), you will
+need to run `./scripts/compile_all_examples.py` to ensure the `examples/sizes.txt` file gets 
+updated.
+
+
+