Document PYTHON_VERSION option

Functionality added in #363

Author: verythorough

included_software.md

@@ -14,7 +14,7 @@ The specific patch versions included will depend on when the image was last buil
 * Node.js - `NODE_VERSION`, `.nvmrc`, `.node-version`
   * 10 (default)
   * Any version that `nvm` can install.
-* Python - `runtime.txt` or `Pipfile`
+* Python - `PYTHON_VERSION`, `runtime.txt`, `Pipfile`
   * 2.7 (default)
   * 3.5
   * 3.7