README.md: add nightly build test status badge

README.md

@@ -18,6 +18,8 @@ We are supporting customers building solutions on AWS with meta-aws. Let us know
 We are yocto project compatible<a href="https://www.yoctoproject.org/software-overview/layers/?searchTerm=meta-aws" target="none" title="What is this?">(?)</a>.
 Currently maintained releases are all [NOT-EOL ones](https://wiki.yoctoproject.org/wiki/Releases). All prior releases will be handled on a case by case basis. Again, please let us know if you're in a crunch on earlier releases and we'll help you the best we can!
 
+[![build-test in all supported branches](https://github.com/aws4embeddedlinux/meta-aws/actions/workflows/build-test-all-branches.yml/badge.svg)](https://github.com/aws4embeddedlinux/meta-aws/actions/workflows/build-test-all-branches.yml)
+
 ### CVE checks
 We do nightly CVE checks on the master branch for every recipe in our layer. You can see those check results here: