Add note about todo keywords to ignore for percent-states

[Thaodan]

I'm using some todo keywords to mark some items as for example a meeting into my agenda view.
When the user forgets to add a timestamp to those org-caldav tries to synchronise them as a (v)todo even thou these are not todos.

This issue can be prevented by only listing the keywords listed in org-percent-states
inside org-icalendar-include-todo.
Maybe org-caldav should either ignored unmapped todos states or only consider
items as todos that are included inside percent-states.

[Thaodan]

This snipped helped my to only include the
todo export those inside org-caldav-todo-percent-states:

(setopt org-icalendar-include-todo
        (cl-loop for (p s) in
                 org-caldav-todo-percent-states collect s))