Add a comment to explain release notes code

.github/workflows/deploy.yml

@@ -34,6 +34,11 @@ jobs:
       run: |
         VERSION=$(cargo pkgid | cut -d# -f2 | cut -d: -f2)
         echo "::set-output name=version::$VERSION"
+        # Steps to extract the last release notes from CHANGES:
+        #   1. Remove the first three lines
+        #   2. Stop at the next heading level
+        #   3. Remove the last line
+        #   4. Deindent the bullet points to avoid a markdown code block
         tail -n +3 CHANGES | sed '/^##/q' |
         sed '$ d' | awk '{$1=$1};1' > CHANGES.txt