go 1.16: installation fails due to incorrect path in installation instructions

[fgierlinger]

Description The installation instruction with go get are the following:

https://github.com/magefile/mage/blob/07afc7d24f4d6d6442305d49552f04fbda5ccb3e/README.md#L19-L21

since go v1.16 go get does not install the package under ${GOPATH}/src but ${GOPATH}/pkg/mod/.

Steps to reproduce

$ sudo docker run --rm -ti golang:1.15 /bin/bash -c "go get -u -d github.com/magefile/mage && go version &&  find /go -type f -name 'bootstrap.go'"
go version go1.15.8 linux/arm64
/go/src/github.com/magefile/mage/bootstrap.go

$ sudo docker run --rm -ti golang:1.16 /bin/bash -c "go get -u -d github.com/magefile/mage && go version &&  find /go -type f -name 'bootstrap.go'"
go: downloading github.com/magefile/mage v1.11.0
go version go1.16 linux/arm64
/go/pkg/mod/github.com/magefile/[email protected]/bootstrap.go