CI: Add separate contributors action (#2689)

.github/workflows/contributor.yml

@@ -0,0 +1,17 @@
+# DESCRIPTION: Github actions config
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+name: Contributor Agreement
+
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  Test:
+    name: "'docs/CONTRIBUTORS' was signed"
+    runs-on: ubuntu-20.04
+    steps:
+    - uses: actions/checkout@v2
+    - run: test_regress/t/t_dist_contributors.pl