Add a reminder

We need a reminder add the step-security/harden-runner action at the
top of every job.

Co-authored-by: Nick <50747025+mcdonnnj@users.noreply.github.com>

.github/workflows/build.yml

@@ -18,6 +18,8 @@ jobs:
     name: Run diagnostics
     runs-on: ubuntu-latest
     steps:
+      # Note that a duplicate of this step must be added at the top of
+      # each job.
       - id: harden-runner
         name: Harden the runner
         uses: step-security/harden-runner@v2