add version notes to readme

README.rst

@@ -72,3 +72,10 @@ Questions can also be asked on
 
 .. image:: https://github.com/rwth-i6/returnn/workflows/CI/badge.svg
     :target: https://github.com/rwth-i6/returnn/actions
+
+Dependencies
+============
+
+pip dependencies are listed in ``requirements.txt`` and ``requirements-dev``, although some parts of the code may require additional dependencies (e.g. ``librosa``, ``resampy``) on-demand.
+
+RETURNN supports Python >= 3.8. Bumps to the minimum Python version are listed in `CHANGELOG.md <https://github.com/rwth-i6/returnn/blob/master/CHANGELOG.md>`__.