Add test case documentation
Add test case documentation

We want people to test every new functionality that gets contributed
as well as cover all functionality in the code base. To make it
easier for contributors to know how, we need to provide documentation
for how to create tests and work with existing ones.

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