clarify download instructions (fixes #44)
clarify download instructions (fixes #44)

fplll/fplll,blowfish880/fplll,fplll/fplll,cr-marcstevens/fplll,dstehle/fplll,shi-bai/fplll,GottfriedHerold/fplll,gbonnoron/fplll,pfasante/fplll,gbonnoron/fplll,GottfriedHerold/fplll,pfasante/fplll,dstehle/fplll,cr-marcstevens/fplll,gbonnoron/fplll,shi-bai/fplll,GottfriedHerold/fplll,fplll/fplll,dstehle/fplll,fplll/fplll,malb/fplll,cr-marcstevens/fplll,malb/fplll,malb/fplll,fplll/fplll,blowfish880/fplll,pfasante/fplll,cr-marcstevens/fplll,shi-bai/fplll,blowfish880/fplll,shi-bai/fplll