Add badges to readme to indicate CI status

Author: per1234

README.md

@@ -1,5 +1,10 @@
 # arduino/compile-sketches action
 
+[![Tests](https://github.com/arduino/compile-sketches/workflows/Test%20Python%20code/badge.svg)](https://github.com/arduino/compile-sketches/actions?workflow=Test+Python+code)
+[![Lint](https://github.com/arduino/compile-sketches/workflows/Lint%20Python%20code/badge.svg)](https://github.com/arduino/compile-sketches/actions?workflow=Lint+Python+code)
+[![Spell Check](https://github.com/arduino/compile-sketches/workflows/Spell%20Check/badge.svg)](https://github.com/arduino/compile-sketches/actions?workflow=Spell+Check)
+[![codecov](https://codecov.io/gh/arduino/compile-sketches/branch/master/graph/badge.svg)](https://codecov.io/gh/arduino/compile-sketches)
+
 This action checks whether Arduino sketches compile and produces a report of data from the compilations.
 
 ## Inputs