docs: document how to use multiple Python versions

README.md

@@ -86,7 +86,37 @@ dependencies = [
   "pytest>=8.0.0,<9",
 ...
 ]
-````
+```
+
+## Use Multiple Python Versions
+
+For some jobs, it can be relevant to use multiple Python versions, such as:
+
+- Test jobs: `py-unittest`, `py-pytest`, `py-nosetest`
+- `py-publish` jobs (especially if you're not generating a pure Python package)
+
+This setup is done by defining the PYTHON_IMAGE variable with a parallel/matrix strategy in your .gitlab-ci.yml.
+
+For example, to run py-test jobs using both python:3.13-slim and python:3.12-alpine:
+
+```yaml
+py-pytest:
+  parallel:
+    matrix:
+      - PYTHON_IMAGE: python:3.13-slim
+      - PYTHON_IMAGE: python:3.12-alpine
+```
+
+If your tests cannot be executed concurrently due to shared resources (e.g. database access), you can use the `resource_group` feature to limit parallel execution:
+
+```yaml
+py-pytest:
+  parallel:
+    matrix:
+      - PYTHON_IMAGE: python:3.13-slim
+      - PYTHON_IMAGE: python:3.12-alpine
+  resource_group: db_access_tests
+```
 
 ## Jobs