docs(README): added PyPI version badge

Author: ErikBjare

README.md

@@ -3,6 +3,7 @@ aw-core
 
 [![GitHub Actions badge](https://github.com/ActivityWatch/aw-core/workflows/Build/badge.svg)](https://github.com/ActivityWatch/aw-core/actions)
 [![Code coverage](https://codecov.io/gh/ActivityWatch/aw-core/branch/master/graph/badge.svg)](https://codecov.io/gh/ActivityWatch/aw-core)
+[![PyPI](https://img.shields.io/pypi/v/aw-core)](https://pypi.org/project/aw-core/)
 [![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/)