Update FAQ.md

* add how to create CustomDrawerItems FAQ

FAQ.md

@@ -8,6 +8,7 @@ Each question and it's answer is hosted in a separate file within the FAQ folder
 * [How do I make the drawer appear underneath the Toolbar?](FAQ/howto_show_drawer_under_toolbar.md)
 * [How do I create a Multi-pane layout with MaterialDrawer for tablets?](FAQ/howto_show_drawer_in_tablet_multipane.md)
 * [Why is there whitespace where the status bar should be while my theme is set as fullscreen ?](FAQ/status_bar_whitespace.md)
+* [How do I change existing or add my own CustomDrawerItems to the MaterialDrawer?](FAQ/howto_modify_add_custom_draweritems.md)
 
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