Add introduction to the new online help system

This adds a very basic explanation of the new help command that can be
displayed using ":help intro". This introduction is also added as
default page so that users can simply type ":help" instead.

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

doc/Makefile.am

@@ -29,6 +29,7 @@ dist_doc_DATA = \
 	general.txt \
 	goto.txt \
 	import.txt \
+	intro.txt \
 	main.txt \
 	manual.txt \
 	other.txt \

doc/intro.txt

@@ -0,0 +1,15 @@
+calcurse Online Help
+====================
+
+Welcome to the calcurse online help. The online help system allows for easily
+getting help on specific calcurse features.
+
+On the calcurse main screen, type `:help <feature>` (e.g. `:help add`) to get
+help on a specific feature. Type `:help` without any parameter to display this
+introduction or simply use the corresponding keyboard shortcut (`?` by
+default).
+
+All help texts are displayed using an external pager. You need to exit the
+pager in order to get back to calcurse (pressing `q` should almost always
+work). The default pager can be changed by setting the PAGER environment
+variable.