Add GitHub Actions build status badge to README

Author: jit89

README.md

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 # PicoTimer
 
+![Compile Check](https://github.com/jit89/PicoTimer/actions/workflows/compile-test.yml/badge.svg)
+
 A lightweight, non-blocking, template-based timer library specifically optimized for the **Raspberry Pi Pico (RP2040)** using the [Earle Philhower](https://github.com/earlephilhower/arduino-pico) Arduino Core.
 
 ## Overview