alice: CONTRIBUTING: Mention Python 3.9 requirement

Author: pdxjohnny

entities/alice/CONTRIBUTING.rst

@@ -5,6 +5,10 @@ This document describes how to write Open Architecture overlays,
 overlays for Alice, how to work within the codebase, and the
 contribution process.
 
+Alice currently only supports **Python 3.9 on Linux**. ``pyenv``
+https://github.com/pyenv/pyenv#installation is a good tool to use
+to install another version of Python if your distro doen't have 3.9.
+
 Alice is very much a work in progress. See
 https://github.com/intel/dffml/pull/1401 for status.