Add doc for customizing hook.

docs/customizing_page_views.md

@@ -94,3 +94,28 @@ rails generate administrate:views:layout
  # It only generates the sidebar partial
  # -> app/views/admin/application/_navigation.html.erb
 ```
+
+## Customizing for a specific layout
+
+You can use several hook points to add elements to specific layouts or specific pages.
+
+- header_middle
+- header_last
+- before_main
+- main
+- after_main
+
+For example, you can add a button in the middle of the header as follows.
+
+```eruby
+<%# app/views/admin/customers/_index_header.html.erb %>
+
+<% content_for(:header_middle) do %>
+  <div>
+    You are logged in as <em><%= pundit_user.name %></em>.
+    <%= link_to("Become the Admin", become_admin_customer_path("admin")) unless pundit_user.admin? %>
+  </div>
+<% end %>
+
+<%= render template: 'administrate/application/_index_header', locals: local_assigns %>
+```