chore: add CONTRIBUTING.md, allow running pytest directly in dev

Co-authored-by: Fabio Ambauen <1833932+open-dynaMIX@users.noreply.github.com>

CONTRIBUTING.md

@@ -0,0 +1,21 @@
+# Contributing guide
+
+To contribute to this project, you'll need [Poetry](https://python-poetry.org/).
+
+```sh
+poetry install --with dev
+
+# run tests
+poetry run pytest
+```
+
+We're also using [tox](https://tox.wiki) to run the tests in all supported environments. Running the full test suite is simple:
+
+```sh
+tox
+```
+
+If you'd like to run tests in a specific environment, use `-e`:
+```sh
+tox -e django42
+```