Add a readme

This will provide a quick overview of the project.

The URLs in the "Compile Examples" workflow status badge are configured for the final location of the library, so they are expected to be broken while it is in the temporary development location.

Author: per1234

README.md

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+# `Arduino_Threads` library for Arduino
+
+[![Compile Examples status](https://github.com/arduino-libraries/Arduino_Threads/actions/workflows/compile-examples.yml/badge.svg)](https://github.com/arduino-libraries/Arduino_Threads/actions/workflows/compile-examples.yml)
+
+This library makes it easy to use the multi-threading capability of [Arduino](https://www.arduino.cc/) boards that use an [Mbed OS](https://os.mbed.com/docs/mbed-os/latest/introduction/index.html)-based core library.