[TASK] Document the general contribution workflow

Part of #489

.github/CONTRIBUTING.md

@@ -10,3 +10,25 @@ When you contribute, please take the following things into account:
 Please note that this project is released with a
 [Contributor Code of Conduct](../CODE_OF_CONDUCT.md). By participating in this
 project, you agree to abide by its terms.
+
+## General workflow
+
+This is the workflow for contributing changes to this project::
+
+1. [Fork the git repository](https://docs.github.com/en/get-started/exploring-projects-on-github/contributing-to-a-project).
+2. Clone your forked repository locally and install the development dependencies.
+3. Add a local remote "upstream" so you will be able to
+   [synchronize your fork with the original repository](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork).
+4. Create a local branch for your changes.
+5. Add unit tests for your changes.
+   These tests should fail without your changes.
+6. Add your changes. Your added unit tests now should pass, and no other tests
+   should be broken. Check that your changes follow the same coding style as the
+   rest of the project.
+7. Add a changelog entry.
+8. Commit and push your changes.
+9. [Create a pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests)
+   for your changes. Check that the CI build is green. (If it is not, fix the
+   problems listed.)
+10. Wait for a review by the maintainers.
+11. Polish your changes as needed until they are ready to be merged.