CONTRIBUTING: add basic getting started

Author: dlech

CONTRIBUTING.md

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+# Contributing to pybricksdev
+
+`pybricksdev` is open source software and we welcome contributions.
+
+
+## Developer environment
+
+This is the preferred workflow that we support. If you have other tools you like
+to use, you are welcome to use them, but we can't help you if things go wrong.
+
+### Prerequisites
+
+- [Git](https://git-scm.com/) (or install `git` with your favorite package manager)
+- [Python 3.8](https://python.org) (see [README](./README.md) for more info)
+- [Poetry](https://python-poetry.org/)
+- [VS Code](https://code.visualstudio.com/)
+
+### Get the code and setup the environment
+
+After installing the prerequisites, use a command prompt in the location of your
+choice to run the following:
+
+    git clone https://github.com/pybricks/pybricksdev
+    cd pybricksdev
+    poetry env use python3.8
+    poetry install
+    code .
+
+On Windows, you may need to use the full path to `python3.8` (run `py -0p` to find it).
+
+You will need to tell VS Code to use `.venv` as the Python interpreter. It
+should ask you about this the first time you open the project folder.
+
+You should also opt into the [DeprecatePythonPath] experiment for the Microsoft
+Python VS Code extension so that it does not modify the `.vscode/settings.json`
+file in the project when you select the interpreter.
+
+[DeprecatePythonPath]: https://github.com/microsoft/vscode-python/wiki/AB-Experiments#deprecatepythonpath