Add workflow to create a binder badge comment on PRs

See https://github.com/jupyterhub/binder/issues/212#issuecomment-687706341

.github/workflows/binder-badge.yml

+name: binder-badge
+on:
+  pull_request_target:
+
+jobs:
+  badge:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: manics/action-binderbadge@main
+        with:
+          githubToken: ${{ secrets.GITHUB_TOKEN }}