Update vagrant documentation to include backend workers.
Update vagrant documentation to include backend workers.

The celery workers need to be started as well and the information
about how to do that was missing from the installation documentation.

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