Add changenote

Author: angelapwen

CHANGELOG.md

@@ -6,7 +6,7 @@ Note that the only difference between `v2` and `v3` of the CodeQL Action is the
 
 ## [UNRELEASED]
 
-No user facing changes.
+- On Linux, the maximum possible value for the `--threads` option now respects the CPU count as specified in `cgroup` files to more accurately reflect the number of available cores when running in containers. 
 
 ## 3.23.1 - 17 Jan 2024