Update README.md

Author: matthias-bs

README.md

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+[![formatting: lint](https://github.com/matthias-bs/MicroPython-ADC_Cal/actions/workflows/ci.yml/badge.svg)](https://github.com/matthias-bs/MicroPython-ADC_Cal/actions/workflows/ci.yml)
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 # MicroPython-ADC_Cal
 **MicroPython ESP32 library for calibrated on-chip ADC conversion**