Add a github action to verify PR descriptions (#4971)

This action just verifies that PRs contain descriptions. Our policy
requires PR descriptions, but we don't have consistent application of
the policy. This should help correct that.

.github/workflows/pr-description-checker.yml

@@ -0,0 +1,18 @@
+name: 'PR description checker'
+on:
+  pull_request:
+    types:
+      - opened
+      - edited
+      - reopened
+      - labeled
+      - unlabeled
+jobs:
+  check-pr-description:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: jadrol/[email protected]
+        id: description-checker
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}