add explanatory comments with links to DR security gallery workflow template file

code-scanning/dependency-review.yml

@@ -1,3 +1,9 @@
+# Dependency Review Action
+#
+# This Action will scan dependency manifest files that change as part of a Pull Reqest, surfacing known-vulnerable versions of the packages declared or updated in the PR. Once installed, if the workflow run is marked as required, PRs introducing known-vulnerable packages will be blocked from merging.
+#
+# Source repository: https://github.com/actions/dependency-review-action
+# Public documentation: https://docs.github.com/en/code-security/supply-chain-security/understanding-your-software-supply-chain/about-dependency-review#dependency-review-enforcement
 name: 'Dependency Review'
 on: [pull_request]