Export online help as text files

Move the texts of each online help screen to a separate text file. The
set of files is supposed to replace the online help screens later.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

doc/add.txt

@@ -0,0 +1,37 @@
+Add
+===
+
+Add an item in either the TODO or Appointment list, depending on which
+panel is selected when you press 'a'.
+
+To enter a new item in the TODO list, you will need first to enter the
+description of this new item. Then you will be asked to specify the todo
+priority. This priority is represented by a number going from 9 for the
+lowest priority, to 1 for the highest one. It is still possible to
+change the item priority afterwards, by using the '+' and '-' keys
+inside the todo panel.
+
+If the APPOINTMENT panel is selected while pressing 'a', you will be
+able to enter either a new appointment or a new all-day long event.
+To enter a new event, press [ENTER] instead of the item start time, and
+just fill in the event description.
+To enter a new appointment to be added in the APPOINTMENT list, you
+will need to enter successively the time at which the appointment
+begins, the appointment length (either by specifying the end time in
+[hh:mm] or the duration in [+hh:mm], [+xxdxxhxxm] or [+mm] format), 
+and the description of the event.
+
+The day at which occurs the event or appointment is the day currently
+selected in the calendar, so you need to move to the desired day before
+pressing 'a'.
+
+Notes:
+     o if an appointment lasts for such a long time that it continues
+       on the next days, this event will be indicated on all the
+       corresponding days, and the beginning or ending hour will be
+       replaced by '..' if the event does not begin or end on the day.
+     o if you only press [ENTER] at the APPOINTMENT or TODO event
+       description prompt, without any description, no item will be
+       added.
+     o do not forget to save the calendar data to retrieve the new
+       event next time you launch Calcurse.