Makefile, README, INSTALL: Improve INSTALL documentation

Makefile, README, INSTALL: Improve INSTALL documentation

Add a 'make help' target to document the features of our Makefile.

Move all documentation about installing from the README to a new
INSTALL file, following GNU coding standards.  See
<https://www.gnu.org/prep/standards/html_node/Releases.html#Releases>.

INSTALL contains minimal information, and defers to 'make help'
for the rest.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>