docs(README): added PyPI version badge

Author: ErikBjare

README.md

@@ -2,6 +2,7 @@ aw-client
 =========
 
 [![GitHub Actions badge](https://github.com/ActivityWatch/aw-client/workflows/Build/badge.svg)](https://github.com/ActivityWatch/aw-client/actions)
+[![PyPI](https://img.shields.io/pypi/v/aw-client)](https://pypi.org/project/aw-client/)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 [![Typechecking: Mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)