add pypi badge

Readme.md

@@ -1,9 +1,15 @@
 # days360
 
 [![Tests](https://github.com/tfeldmann/days360/actions/workflows/tests.yml/badge.svg)](https://github.com/tfeldmann/days360/actions/workflows/tests.yml)
+<a href="https://pypi.org/project/days360/">
+  <img src="https://img.shields.io/pypi/v/days360" title="PyPI Version">
+</a>
 
 Calculates the days between two dates based on the 360 day year used in interest calculations.
-It's planned to add the ISDA and PSA methods in the future.
 
+Planned:
+- ISDA and PSA methods.
+
+Notes:
 - Implementations are based on https://en.wikipedia.org/wiki/360-day_calendar.
 - This library started as a port of this ruby gem: https://github.com/tamaloa/days360/tree/master