README updates: how to compile apsim and dssat, overview of psims, how to obtain data
README updates: how to compile apsim and dssat, overview of psims, how to obtain data

schmidtfederico/psims,schmidtfederico/psims,RDCEP/psims,schmidtfederico/psims,schmidtfederico/psims,RDCEP/psims,RDCEP/psims