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 \