.github: add a check for pull request commits to be signed off

Added a workflow to check the commit messages of a PR to make sure each is
signed off by the author. A repo token is required, here named SECRET_TOKEN,
which has access to read and write PR comments.

Signed-off-by: JSOwens <josephs.owens@gmail.com>

.github/workflows/pr-signoff-check.yml

@@ -0,0 +1,15 @@
+name: Check PR Sign-off
+
+on: [pull_request]
+
+jobs:
+  check-sign-off:
+    name: Write comment if unsigned commits found
+    env:
+      FORCE_COLOR: 1
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: live627/check-pr-signoff-action@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}