[docs,readme] add PyPI monthly downloads badge (#34)

Fixes #32

Author: orireiter

README.md

@@ -1,5 +1,6 @@
 [![PyPi](https://img.shields.io/pypi/v/python-json-logger.svg)](https://pypi.python.org/pypi/python-json-logger/)
 [![PyPI - Status](https://img.shields.io/pypi/status/python-json-logger)](https://pypi.python.org/pypi/python-json-logger/)
+[![PyPI - Downloads](https://img.shields.io/pypi/dm/python-json-logger)](https://pypi.python.org/pypi/python-json-logger/)
 [![Python Versions](https://img.shields.io/pypi/pyversions/python-json-logger.svg)](https://github.com/nhairs/python-json-logger)
 [![License](https://img.shields.io/github/license/nhairs/python-json-logger.svg)](https://github.com/nhairs/python-json-logger)
 ![Build Status](https://github.com/nhairs/python-json-logger/actions/workflows/test-suite.yml/badge.svg)

docs/index.md

@@ -2,6 +2,7 @@
 
 [![PyPi](https://img.shields.io/pypi/v/python-json-logger.svg)](https://pypi.python.org/pypi/python-json-logger/)
 [![PyPI - Status](https://img.shields.io/pypi/status/python-json-logger)](https://pypi.python.org/pypi/python-json-logger/)
+[![PyPI - Downloads](https://img.shields.io/pypi/dm/python-json-logger)](https://pypi.python.org/pypi/python-json-logger/)
 [![Python Versions](https://img.shields.io/pypi/pyversions/python-json-logger.svg)](https://github.com/nhairs/python-json-logger)
 [![License](https://img.shields.io/github/license/nhairs/python-json-logger.svg)](https://github.com/nhairs/python-json-logger)
 ![Build Status](https://github.com/nhairs/python-json-logger/actions/workflows/test-suite.yml/badge.svg)