CONTRIBUTING: new contributor guide

CONTRIBUTING.md

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+**Project:** `pybricks-api`
+
+**Acceptable changes:**
+
+- Fixing typos, spelling, grammar, etc.
+- Clarifying existing documentation.
+- Improving code completion/intellisense.
+
+Other more substantial changes should be discussed first by [opening an issue
+on GitHub][issues]. Proposals for new APIs should be discussed in the
+[pybricks-micropython][pbmp] repository since that is where the new features
+will actually be implemented.
+
+
+[issues]: https://github.com/pybricks/pybricks-api/issues
+[pbmp]: https://github.com/pybricks/pybricks-micropython
+
+
+**Commit messages:**
+
+- Commit messages have a one-line subject, a blank line and a multiline body:
+
+        prefix: subject
+
+        body...
+
+- Use a prefix to categorize the change. Usually this is the path the the file
+  being changed. 
+  - If it is a documentation change, it can be shortened to `doc/<file>` (omit
+    `api` from the path).
+  - If it is an API change (in the `pybricks` folder), write `api/<file>` 
+    instead of `pybricks/<file>`.
+- The subject briefly describes _what_ was changed, e.g. `fix typo`.
+- The body describes _why_ the change was made, e.g. `The word "sensor" was
+  spelled incorrectly`.
+- See the [commit history][commits] for more examples.
+
+[commits]: https://github.com/pybricks/pybricks-api/commits/master
+
+
+**Development environment:**
+
+Prerequisites:
+- [VS Code][vscode] (optional but recommended)
+- [Git][git]
+- [Python 3][python]
+- [Poetry][poetry]
+
+Information on installing prerequisites can be found on the [pybricks-micropython
+wiki][wiki].
+
+Initial setup:
+
+    git clone --recursive https://github.com/pybricks/pybricks-api
+    cd pybricks-api
+    poetry install
+
+Build docs:
+
+    # Linux
+    poetry run make -C doc html
+    xdg-open doc/api/build/html/index.html
+
+    # macOS
+    poetry run make -C doc html
+    open doc/api/build/html/index.html
+
+    # Windows (PowerShell)
+    poetry run doc\make.bat html
+    Invoke-Item doc\api\build\html\index.html
+
+Linting:
+
+    poetry run flake8  # check Python
+    poetry run doc8  # check Restructured Text
+
+[vscode]: https://code.visualstudio.com/
+[git]: https://git-scm.com/
+[python]: https://www.python.org/
+[poetry]: https://python-poetry.org/
+[wiki]: https://github.com/pybricks-micropython/wiki