Everyday git webpage could also mention setgid on dirs

Referring to: http://www.kernel.org/pub/software/scm/git/docs/everyday.html

Under the "CVS-style shared repository" section, I think it should
mention that in addition to making the shared repository writable to
the group, it is also necessary to set the gid bit on all directories,
so future added files also have the correct group.

Regards -- Andy