update link to CONTRIBUTING.md in README.md
update link to CONTRIBUTING.md in README.md

chrisfried/DIM,bhollis/DIM,bhollis/DIM,DestinyItemManager/DIM,DestinyItemManager/DIM,bhollis/DIM,bhollis/DIM,delphiactual/DIM,chrisfried/DIM,DestinyItemManager/DIM,delphiactual/DIM,delphiactual/DIM,delphiactual/DIM,chrisfried/DIM,DestinyItemManager/DIM,chrisfried/DIM