.github: add workflow for adding milestone

Signed-off-by: Kefu Chai <kchai@redhat.com>

.github/milestone.yml

@@ -0,0 +1,6 @@
+base-branch:
+  - "luminous"
+  - "nautilus"
+  - "octopus"
+  - "pacific"
+  - "quincy"

.github/workflows/milestone.yml

@@ -0,0 +1,14 @@
+---
+name: "Pull Request Add Milestone"
+on:
+  pull_request:
+    types:
+      - opened
+jobs:
+  milestone:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: iyu/actions-milestone@v1
+        with:
+          configuration-path: .github/milestone.yml
+          repo-token: "${{ secrets.GITHUB_TOKEN }}"