DOC: Add Downloads badge to README

Says: 68/day.

Author: kernc

README.md

@@ -5,6 +5,7 @@ Backtesting.py
 [![Build Status](https://img.shields.io/github/workflow/status/kernc/backtesting.py/CI/master?style=for-the-badge)](https://github.com/kernc/backtesting.py/actions)
 [![Code Coverage](https://img.shields.io/codecov/c/gh/kernc/backtesting.py.svg?style=for-the-badge)](https://codecov.io/gh/kernc/backtesting.py)
 [![Backtesting on PyPI](https://img.shields.io/pypi/v/backtesting.svg?color=blue&style=for-the-badge)](https://pypi.org/project/backtesting)
+[![PyPI downloads](https://img.shields.io/pypi/dd/backtesting.svg?color=skyblue&style=for-the-badge)](https://pypi.org/project/backtesting)
 
 Backtest trading strategies with Python.