Update styleguide with MODULE.bazel instructions

Avoid https://github.com/bazelbuild/bazel-central-registry/pull/5839#discussion_r2337680802
diff --git a/.gemini/styleguide.md b/.gemini/styleguide.md
index 3bc690e..56352a2 100644
--- a/.gemini/styleguide.md
+++ b/.gemini/styleguide.md
@@ -53,6 +53,11 @@
 
 - If there are any changes to `maintainers` or `repository` fields in `metadata.json` files, please ping @bazelbuild/bcr-maintainers for a manual review.
 
+### MODULE.bazel
+
+- The MODULE.bazel file may contain overrides (e.g. `archive_override`, `git_override`, `local_path_override`). Those will be ignored when the module is used as a dependency, so they are allowed and you can ignore reviewing them.
+- There could be symlinks under `overlay/` directory that points to a `MODULE.bazel` file in the parent directory. The content of the symlink will look like a text file `../MODULE.bazel`, these are allowed and should be ignored.
+
 ---
 
 ## 2) Handy References & Commands (for authors)
@@ -66,7 +71,3 @@
     --registry="file:///path/to/bazel-central-registry" \
     --lockfile_mode=off @<module>//:<target>
   ```
-
-## 3) Additional Notes
-
-- There could be symlinks under `overlay/` directory that points to a `MODULE.bazel` file in the parent directory. The content of the symlink will look like a text file `../MODULE.bazel`, these are allowed and should be ignored.