Have Dependabot offer submodule updates

This extends the current Dependabot configuration so that, in
addition to offering updates for GitHub Actions, it also offers
them for git submodules (for the gitdb direct submodule).

.github/dependabot.yml

@@ -4,3 +4,8 @@ updates:
   directory: "/"
   schedule:
     interval: "weekly"
+
+- package-ecosystem: "gitsubmodule"
+  directory: "/"
+  schedule:
+    interval: "monthly"