Utility does not build

[mmgen]

go version go1.16.6 linux/amd64

$ git clone https://github.com/FiloSottile/zcash-mini
$ cd zcash-mini    
$ make
go install -v -ldflags='-X "main.Version=a2b3504" -X "main.BuildTime=2021-08-26-1120 UTC"' github.com/FiloSottile/zcash-mini
go install: version is required when current directory is not in a module
    Try 'go install github.com/FiloSottile/zcash-mini@latest' to install the latest version
make: *** [Makefile:10: zcash-mini] Error 1

[mmgen]

Utility builds successfully with the following commands:

$ cd zcash-mini
$ go mod init zcash-mini
$ go mod tidy
$ go build -mod=mod

[mmgen]

Commands added to README in #15