README.md - add status badge

Author: DavesCodeMusings

README.md

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 # BTHome-MicroPython
 A MicroPython module to format sensor readings for BTHome BLE advertising payloads.
 
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 ## What is it?
 BTHome-MicroPython provides an easy way to send sensor readings from MicroPython-based microcontrollers, via Bluetooth Low Energy (BLE) advertisements, to home automation systems like Home Assistant that support the BTHome data format.