gitweb: Add info about $projectroot and $projects_list to gitweb/README

gitweb: Add info about $projectroot and $projects_list to gitweb/README

Those two configuration variables are important enough that it is
worth to explicitely write about them in the "Gitweb config file
variables" section even if they are usually set during build by
GITWEB_PROJECTROOT and GITWEB_LIST build (Makefile) configuration
variables.

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