gitweb_config.perl: mention display-only ctags setting
In an environment where the web server software does not have
write access to the project's ctags directory it will not be
possible to add ctags.
However, they can still be displayed just fine while omitting
the "add:" box.
Mention this option explicitly to make it more discoverable.
The projtool.pl addtags command can still be used to easily
add tags to a project without requiring web server support
when using the display only option.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>