Add badges to readme to indicate CI status

Author: per1234

README.md

@@ -1,5 +1,9 @@
 # `arduino/report-size-deltas` action
 
+[![Tests](https://github.com/arduino/report-size-deltas/workflows/libraries/report-size-deltas%20workflow/badge.svg)](https://github.com/arduino/report-size-deltas/actions?workflow=libraries/report-size-deltas+workflow)
+[![Spell Check](https://github.com/arduino/report-size-deltas/workflows/Spell%20Check/badge.svg)](https://github.com/arduino/report-size-deltas/actions?workflow=Spell+Check)
+[![codecov](https://codecov.io/gh/arduino/report-size-deltas/branch/master/graph/badge.svg)](https://codecov.io/gh/arduino/report-size-deltas)
+
 This action comments on the pull request with a report on the resulting change in memory usage of the [Arduino](https://www.arduino.cc/) sketches compiled by the [`arduino/compile-sketches`](https://github.com/arduino/compile-sketches) action. This should be run from a [scheduled workflow](https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#onschedule).
 
 ## Inputs