Add new workflow for greetings

Create one new GitHub workflow named greetings. When community users submit
their first time issue or PR, the workflow runs and comments with
customized greetings to encourage them.

.github/workflows/greetings.yml

@@ -0,0 +1,16 @@
+name: Greetings
+
+on: [pull_request_target, issues]
+
+jobs:
+  greeting:
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+      pull-requests: write
+    steps:
+    - uses: actions/first-interaction@v1
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        issue-message: "Hey, @${{ github.actor }} welcome!🎊 Thanks for taking the time to point this out.🙌"
+        pr-message: "Hiiii, @${{ github.actor }} welcome!🎊 Thanks for taking the effort to make our project better! 🙌 Keep making such awesome contributions!"