ci: add PR comment and cc reviewers

.github/workflows/welcome.yaml

@@ -0,0 +1,40 @@
+name: Welcome
+
+on:
+  pull_request_target:
+    types: [opened]
+
+jobs:
+  comment:
+    permissions:
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - name: Comment
+        uses: peter-evans/create-or-update-comment@v3
+        with:
+          issue-number: ${{ github.event.pull_request.number }}
+          body: |
+            🚀 Thank you for contributing to the [Envoy Gateway](https://gateway.envoyproxy.io/) project! 🚀
+
+            Before merging, please ensure to follow the process below:
+
+            1. Requesting Reviews:
+            - cc @envoyproxy/gateway-reviewers team for an initial review.
+            - After the initial review, reviewers should request the @envoyproxy/gateway-maintainers team for further review.
+            2. Review Approval:
+            - Your PR needs to receive at least two approvals.
+            - At least one approval must come from a member of the gateway-maintainers team.
+
+            **NOTE**: Once your PR is under review, ***please do not rebase and force push it***. Otherwise, it will force your reviewers to review the PR from scratch rather than simply look at your latest changes.
+
+            <details>
+            <summary> What's more, you can help expedite the processing of your PR by </summary>
+            <br>
+
+            - Ensuring you have self-reviewed your work according to the project's [Contribution Guidelines](https://gateway.envoyproxy.io/latest/contributions/develop).
+            - If your PR addresses a specific issue, make sure to mention it in the PR description.
+            - Respond promptly if there are any test failures or suggestions for improvements that we comment on.
+
+            </details>
+          reactions: 'heart'