Add notes to CONTRIBUTING about build badge branches

CONTRIBUTING.md

@@ -12,6 +12,7 @@ If you want to add an entry to the `README.md` please consider this:
 
 - is the entry valuable to people trying to get things done in Rust?
 - if you want to add something, please use the template `[YYY](https://crates.io/crates/YYY) — DESCRIPTION [<img src="https://api.travis-ci.org/XXX/YYY.svg?branch=master">](https://travis-ci.org/XXX/YYY)`
+    * If you're adding a build badge please make sure to add the branch information to the image (as per the example above for Travis). For Github actions please see https://docs.github.com/en/actions/configuring-and-managing-workflows/configuring-a-workflow#example-using-the-branch-parameter
 - please pay attention to the alphabetical ordering.