Fixes #216: mention celery in install docs
Fixes #216: mention celery in install docs

GreenLunar/Bookie,skmezanul/Bookie,charany1/Bookie,GreenLunar/Bookie,pombredanne/Bookie,wangjun/Bookie,wangjun/Bookie,pombredanne/Bookie,skmezanul/Bookie,bookieio/Bookie,adamlincoln/Bookie,teodesson/Bookie,pombredanne/Bookie,teodesson/Bookie,adamlincoln/Bookie,teodesson/Bookie,bookieio/Bookie,adamlincoln/Bookie,wangjun/Bookie,teodesson/Bookie,skmezanul/Bookie,charany1/Bookie,adamlincoln/Bookie,bookieio/Bookie,charany1/Bookie,wangjun/Bookie,GreenLunar/Bookie,GreenLunar/Bookie,skmezanul/Bookie,bookieio/Bookie