add note on versioning approach to changelog

Author: nickfyson

.github/workflows/post-release-mergeback.yml

@@ -133,8 +133,8 @@ jobs:
           # Update the version number ready for the next release
           npm version patch --no-git-tag-version
 
-          # Update the changelog
-          perl -i -pe 's/^/## \[UNRELEASED\]\n\nNo user facing changes.\n\n/ if($.==5)' CHANGELOG.md
+          # Update the changelog, adding a new version heading directly above the most recent existing one
+          awk '!f && /##/{print "'"## [UNRELEASED]\n\nNo user facing changes.\n"'"; f=1}1' CHANGELOG.md > temp && mv temp CHANGELOG.md
           git add .
           git commit -m "Update changelog and version after ${VERSION}"
 

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 See the [releases page](https://github.com/github/codeql-action/releases) for the relevant changes to the CodeQL CLI and language packs.
 
+Note that for the CodeQL Action we use the major version number to mark a change in the node version used, while the minor & patch numbers indicate releases that support the same features across node versions. For example `3.22.11` is the first `v3` release and is functionally identical to `2.22.11`, which was released while we continue to support `v2`. This approach ensures an easy way to track exactly which features are included in different versions.
+
 ## [UNRELEASED]
 
 No user facing changes.