README: add downloads badge

Author: paroj

README.md

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 sensors.py
 ==========
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+[![Downloads](https://pepy.tech/badge/sensors-py)](https://pepy.tech/project/sensors-py)
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 python bindings using ctypes for libsensors3 of the [lm-sensors project](https://github.com/lm-sensors/lm-sensors). The code was written against libsensors 3.3.4.  
 
 For documentation of the low level API see [sensors.h](https://github.com/lm-sensors/lm-sensors/blob/master/lib/sensors.h). For an example of the high level API see [example.py](https://github.com/paroj/sensors.py/blob/master/example.py).