Have Dependabot offer submodule updates

EliahKagan · EliahKagan · commit 7b1c04681a7d · 2023-10-13T02:05:03.000-04:00
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).
diff --git a/.github/dependabot.yml b/.github/dependabot.yml
@@ -4,3 +4,8 @@ updates:
   directory: "/"
   schedule:
     interval: "weekly"
+
+- package-ecosystem: "gitsubmodule"
+  directory: "/"
+  schedule:
+    interval: "monthly"
