Add badge about python versions

Author: rlan

README.md

@@ -1,6 +1,7 @@
 # A Charles Schwab CSV Converter for Portfolio Performance
 
 [![Testing badge](https://github.com/rlan/convert-csv-schwab2pp/actions/workflows/python-app.yml/badge.svg)](https://github.com/rlan/convert-csv-schwab2pp/actions)
+![Python versions](https://img.shields.io/badge/python-3.7%20%7C%203.8%20%7C%203.9%20%7C%203.10%20%7C%203.11%20%7C%203.12-blue)
 ![MIT license](https://img.shields.io/github/license/rlan/convert-csv-schwab2pp)
 
 Converts a [Charles Schwab](https://www.schwab.com/) transaction CSV file to a ready-to-import CSV file for [Portfolio Performance](https://www.portfolio-performance.info/en/) (PP).