docs: mention Varnish support in install guide.
docs: mention Varnish support in install guide.

seabirdzh/disco,scrapinghub/disco,ErikDubbelboer/disco,pooya/disco,simudream/disco,seabirdzh/disco,beni55/disco,simudream/disco,beni55/disco,pooya/disco,simudream/disco,mozilla/disco,simudream/disco,pavlobaron/disco_playground,seabirdzh/disco,ErikDubbelboer/disco,ErikDubbelboer/disco,beni55/disco,oldmantaiter/disco,discoproject/disco,mwilliams3/disco,seabirdzh/disco,ktkt2009/disco,discoproject/disco,pombredanne/disco,mozilla/disco,beni55/disco,seabirdzh/disco,discoproject/disco,mozilla/disco,pombredanne/disco,simudream/disco,oldmantaiter/disco,mozilla/disco,oldmantaiter/disco,mwilliams3/disco,scrapinghub/disco,scrapinghub/disco,ktkt2009/disco,ErikDubbelboer/disco,pavlobaron/disco_playground,mwilliams3/disco,beni55/disco,mwilliams3/disco,oldmantaiter/disco,scrapinghub/disco,ktkt2009/disco,discoproject/disco,ktkt2009/disco,pombredanne/disco,pombredanne/disco,discoproject/disco,pooya/disco,pombredanne/disco,pooya/disco,mwilliams3/disco,oldmantaiter/disco,pavlobaron/disco_playground,pavlobaron/disco_playground,ktkt2009/disco,ErikDubbelboer/disco