Add instructions for the course menu

[markkuriekkinen]

The course menu in the left-side bar can contain links to LTI services or just plain links. Links that point into the same A+ course can be added too and it is rather confusing.

Links into the same A+ course:
In the "add menu item" form, do not select any service and write the menu url relatively to the course path. E.g., modulekey/chapterkey without / at the start.