document AddIndex() in README.md
document AddIndex() in README.md

kevinc0825/gorm,GeertJohan/gorp,gabelula/gorp,go-gorp/gorp,cortex/gorp,gabelula/gorp,mousesun/gorp,yp-engineering/gorp,mousesun/gorp,yp-engineering/gorp,cortex/gorp,kevinc0825/gorm,go-gorp/gorp,GeertJohan/gorp