docs(git-submodules): example of custom per-submodule checks (#12350)

Co-authored-by: Rhys Arkins <rhys@arkins.net>

lib/manager/git-submodules/readme.md

@@ -2,3 +2,14 @@ Keeps publicly accessible Git submodules updated within a repository.
 
 Renovate does not support updating Git submodules that are hosted on a private repository.
 Subscribe to [issue #10149 on GitHub](https://github.com/renovatebot/renovate/issues/10149) to keep track of our progress towards supporting private Git submodules.
+
+You can customize the per-submodule checks of the git-submodules manager like this:
+
+```json
+{
+  "ignoreDeps": ["path/to/submodule", "path/to/submodule2"],
+  "git-submodules": {
+    "enabled": true
+  }
+}
+```