Update README.md (PyPI downloads)

Author: dmitry-lipetsk

README.md

@@ -1,6 +1,7 @@
 [![Build Status](https://app.travis-ci.com/postgrespro/testgres.common.svg?token=HgEMhyw9n7RCQkHCis4T&branch=master)](https://app.travis-ci.com/postgrespro/testgres.common)
 [![PyPI package version](https://badge.fury.io/py/testgres_common.svg)](https://badge.fury.io/py/testgres.common)
 [![PyPI python versions](https://img.shields.io/pypi/pyversions/testgres_common)](https://pypi.org/project/testgres.common)
+[![PyPI downloads](https://img.shields.io/pypi/dm/testgres_common)](https://pypi.org/project/testgres.common)
 
 # testgres - common