gitweb: Add some installation notes in gitweb/INSTALL

Add some installation and configuration notes for gitweb in
gitweb/INSTALL. Make use of filling gitweb configuration by
Makefile.

It does not cover (yet?) all the configuration variables and
options.

Some of contents duplicates information in gitweb/README file
(it is referred from gitweb/INSTALL).

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>