readme: add note about minimum go version

[missinglink]

Prior to go1.16 the make cli command emits the following errors:

make cli
go build -mod vendor -o bin/assign-geometry cmd/assign-geometry/main.go
# github.com/whosonfirst/go-ioutil
vendor/github.com/whosonfirst/go-ioutil/readseekcloser.go:27:41: undefined: io.ReadSeekCloser
vendor/github.com/whosonfirst/go-ioutil/readseekcloser.go:34:7: undefined: io.ReadSeekCloser
vendor/github.com/whosonfirst/go-ioutil/readseekcloser.go:35:14: undefined: io.ReadSeekCloser
vendor/github.com/whosonfirst/go-ioutil/readseekcloser.go:110:15: undefined: io.ReadAll
# github.com/whosonfirst/go-writer
vendor/github.com/whosonfirst/go-writer/file.go:74:19: undefined: os.CreateTemp
vendor/github.com/whosonfirst/go-writer/null.go:29:17: undefined: io.Discard
make: *** [cli] Error 2

[thisisaaronland]

Thanks.