Add downloads badge (#418)

We're gaining momentum, so why not show this in form of a badge

README.md

@@ -7,6 +7,7 @@
 
 [![Supports Python](https://img.shields.io/pypi/pyversions/baybe?style=flat-square&label=Supports%20Python&labelColor=96d7d2&color=ffdcb9)](https://pypi.org/project/baybe/)
 [![PyPI version](https://img.shields.io/pypi/v/baybe.svg?style=flat-square&label=PyPI%20Version&labelColor=96d7d2&color=ffdcb9)](https://pypi.org/project/baybe/)
+[![Downloads](https://img.shields.io/pypi/dm/baybe?style=flat-square&label=Downloads&labelColor=96d7d2&color=ffdcb9)](https://pypistats.org/packages/baybe)
 [![Issues](https://img.shields.io/github/issues/emdgroup/baybe?style=flat-square&label=Issues&labelColor=96d7d2&color=ffdcb9)](https://github.com/emdgroup/baybe/issues/)
 [![PRs](https://img.shields.io/github/issues-pr/emdgroup/baybe?style=flat-square&label=PRs&labelColor=96d7d2&color=ffdcb9)](https://github.com/emdgroup/baybe/pulls/)
 [![License](https://shields.io/badge/License-Apache%202.0-green.svg?style=flat-square&labelColor=96d7d2&color=ffdcb9)](http://www.apache.org/licenses/LICENSE-2.0)