Makefile: Fix the source file name

When we renamed :pacman -> :pacapt, we forgot to update the
Makefile, so the :README section will not work
Re-add INSTALL section in :_help

For some reason, this section was removed in
 #418179574cf6152ac99f9a5cb647885d7bca3d66
Due to the name change (:pacman -> :pacapt), and the error
of :Makefile, the removal has no effect: The README file
is not update-to-date.