installing-boot9strap-(mset9-cli): recommend against Microsoft Store

The version of Python from the Microsoft Store does not work with MSET9,
in part because this version doesn't include the "py" launcher, which is
what MSET9-Windows.bat wants. I don't know if it does actually work if
mset9.py is used directly though, but until then, the python.org
installers should be recommended.

docs/installing-boot9strap-(mset9-cli).md

@@ -18,6 +18,7 @@ This page requires a computer running Windows, Linux, or macOS. If you have an A
 
 * The latest release of [MSET9](https://github.com/hacks-guide/MSET9/releases/latest)
 * Any 3.x version of [Python](https://www.python.org/downloads/) **installed on your computer**
+    + If you are on Windows, Python from the Microsoft Store cannot be used. Please install a version from python.org.
     + If you are on Linux or macOS, you may already have Python 3. Check by opening a Terminal window and entering `python3 -V`. If this returns a version number, it will work for this guide.
 
 ## Instructions