Add require label workflow (#19)

## What does this change?

This repo was missing the require-label workflow. So we've added it in
as part of the CORE4 enforcement

Author: coldlink

.github/workflows/require-label.yml

@@ -0,0 +1,9 @@
+name: CORE4 label enforcement
+permissions:
+  contents: read
+on:
+  pull_request:
+    types: [opened, labeled, unlabeled, synchronize, reopened, edited]
+jobs:
+  require-label:
+    uses: guardian/.github/.github/workflows/require-label.yaml@6dbd851a2512260c2ad854b62c170e2919b7005c #  v3.1.1