DOC: Add CI badge

Author: pllim

README.md

@@ -1,5 +1,6 @@
 # AIDA, the Astronomical Image Display API
 
 [![Documentation Status](https://readthedocs.org/projects/aida/badge/?version=latest)](https://aida.readthedocs.io/en/latest/?badge=latest)
+[![CI](https://github.com/astropy/astro-image-display-api/actions/workflows/run_tests.yml/badge.svg)](https://github.com/astropy/astro-image-display-api/actions/workflows/run_tests.yml)
 
 Also see: https://github.com/astropy/astrowidgets