Update README.md (PyPI downloads)

Author: dmitry-lipetsk

README.md

@@ -2,6 +2,7 @@
 [![codecov](https://codecov.io/gh/postgrespro/testgres/branch/master/graph/badge.svg)](https://codecov.io/gh/postgrespro/testgres)
 [![PyPI package version](https://badge.fury.io/py/testgres.svg)](https://badge.fury.io/py/testgres)
 [![PyPI python versions](https://img.shields.io/pypi/pyversions/testgres)](https://pypi.org/project/testgres)
+[![PyPI downloads](https://img.shields.io/pypi/dm/testgres)](https://pypi.org/project/testgres)
 
 [Documentation](https://postgrespro.github.io/testgres/)