Add GH action to assign PR to its author

.github/workflows/pullrequest-opened.yml

@@ -0,0 +1,13 @@
+name: Pull Request Opened
+on:
+  pull_request_target:
+    types: [opened]
+
+jobs:
+  assign:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Assign Pull Request to its Author
+        uses: technote-space/assign-author@v1
+        with:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}