[TASK] Document the general contribution workflow (#662)

Part of #489

.github/CONTRIBUTING.md

@@ -10,3 +10,26 @@ 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).
+1. Clone your forked repository locally and install the development
+   dependencies.
+1. Create a local branch for your changes.
+1. Add unit tests for your changes.
+   These tests should fail without your changes.
+1. 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.
+1. Add a changelog entry, newest on top.
+1. Commit and push your changes.
+1. [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.
+1. Check that the CI build is green. (If it is not, fix the problems listed.)
+   Please note that for first-time contributors, you will need to wait for a
+   maintainer to allow your CI build to run.
+1. Wait for a review by the maintainers.
+1. Polish your changes as needed until they are ready to be merged.