Add Dependabot (openPMD#929)

Add the new GitHub dependabot service.
Start with GH Action dependencies.

.github/dependabot.yml

@@ -0,0 +1,19 @@
+# Reference:
+#   https://help.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    ignore:
+      # Official actions have moving tags like v1
+      # that are used, so they don't need updates here
+      - dependency-name: "actions/checkout"
+#      - dependency-name: "actions/setup-python"
+      - dependency-name: "actions/cache"
+#      - dependency-name: "actions/upload-artifact"
+#      - dependency-name: "actions/download-artifact"
+#      - dependency-name: "actions/labeler"