add CONTRIBUTING.md

CONTRIBUTING.md

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+# getting started
+
+## with asdf
+
+### tools needed
+
+- python 3.11
+- [hatch](https://hatch.pypa.io/)
+- [asdf](https://asdf-vm.com/guide/getting-started.html)
+
+### setup
+
+1. install [asdf](https://asdf-vm.com/guide/getting-started.html)
+2. run `asdf install` in this directory to install all needed tools
+3. run `just setup` to install the pre-commit hooks etc.
+
+### pre-commit
+
+1. run `just check` to lint the files and auto-format them.
+   you can optionally run `just format` and `just lint` as a single action.
+2. fix the issues which ruff reports
+3. run `just build` to check if it builds correctly
+4. commit changes
+
+## manual
+
+### tools needed
+
+- python 3.11
+- [hatch](https://hatch.pypa.io/)
+
+### setup
+
+1. install [just](https://github.com/casey/just)
+2. run `just setup` to install the pre-commit hooks etc.
+
+### pre-commit
+
+1. run `hatch run lint:fmt` to lint the files and auto-format them.
+2. fix the issues which ruff reports
+3. run `hatch build --clean` to check if it builds correctly
+4. commit changes