docs: describe the site menu in README

Assisted-by: Delta:zai-glm-5.3

README.md

@@ -54,6 +54,10 @@ Sidebar labels come from the page's `#metadata(...)` line, not from the config.
    `#metadata((title: "Overview", ...))` for its sidebar label.
 2. Add a `[[sidebar.section]]` named after the category in `calepin.toml`,
    listing the index page and its sub-pages.
+3. Add a `[[menus.main]]` entry with the category's `index.typ` as `target`
+   and the category name as `label` (e.g. `Home`, `IT`), so it appears in the
+   top navigation bar. The `label` is required here because the index page's
+   metadata title is `Overview`.
 
 ## Deployment