docs: Add lockdown workflow

Add workflow to automatically close any issue or pull request

.github/workflows/lockdown.yaml

@@ -0,0 +1,31 @@
+name: 'Repo Lockdown'
+
+on:
+  issues:
+    types: opened
+  pull_request_target:
+    types: opened
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  action:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/repo-lockdown@v4
+        with:
+          github-token: ${{ github.token }}
+          issue-comment: |
+              Thank you for opening an issue however,
+              PiVPN is in read-only mode, issues are not accepted.
+              More info is available in the README.
+              Github discussions is still available if you want help from community.
+
+          pr-comment: |
+              Thank you for your pull request however,
+              PiVPN is in read-only mode, pull requests are not accepted.
+              More info is available in the README.
+              Github discussions is still available if you want help from community.
+