Add badge for "Check Certificates" workflow to readme

The badges provide a prominent indication of the repository's CI status at a glance. This may help to bring potential
issues to the attention of the maintainers.

Author: per1234

README.md

@@ -4,6 +4,7 @@
 [![Nightly Status](https://github.com/arduino/arduino-lint/workflows/Nightly%20build/badge.svg)](https://github.com/arduino/arduino-lint/actions?workflow=Nightly+build)
 [![Docs Status](https://github.com/arduino/arduino-lint/workflows/Publish%20documentation/badge.svg)](https://github.com/arduino/arduino-lint/actions?workflow=Publish+documentation)
 [![Codecov](https://codecov.io/gh/arduino/arduino-lint/branch/main/graph/badge.svg?token=nprqPQMbdh)](https://codecov.io/gh/arduino/arduino-lint)
+[![Check Certificates status](https://github.com/arduino/arduino-lint/actions/workflows/check-certificates.yml/badge.svg)](https://github.com/arduino/arduino-lint/actions/workflows/check-certificates.yml)
 
 **Arduino Lint** is a command line tool that checks for common problems in [Arduino](https://www.arduino.cc/) projects: