update and (hopefully) improve the directors dev guide
update and (hopefully) improve the directors dev guide

Much more could be said, but I hope this helps understand the
overall concepts a bit better.

I've also taken the freedom to reword "cluster" in the director
concept to "load balancer" because at least in my experience, the
former is used more for some form of tighter binding between nodes,
while the latter is a common term for what directors usually do.

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache