Add deployment recommendations about celery into the docs
Add deployment recommendations about celery into the docs

Celery now fetches the broker url from an environment variable so
the deployment documentation should reflect that.

CivicVision/datahub,openspending/spendb,spendb/spendb,johnjohndoe/spendb,openspending/spendb,USStateDept/FPA_Core,pudo/spendb,USStateDept/FPA_Core,johnjohndoe/spendb,spendb/spendb,CivicVision/datahub,openspending/spendb,pudo/spendb,USStateDept/FPA_Core,CivicVision/datahub,nathanhilbert/FPA_Core,johnjohndoe/spendb,spendb/spendb,nathanhilbert/FPA_Core,pudo/spendb,nathanhilbert/FPA_Core