CLI Install instructions are incorrect for Go 1.16+ #603

Describe the Bug
The CLI instruction list this step for go 1.16+ https://github.com/golang-migrate/migrate/tree/master/cmd/migrate
go install -tags 'postgres' -o $GOPATH/bin/migrate github.com/golang-migrate/migrate/v4/cmd/migrate@$TAG
However the go install command has no -o option.

Also the initial go get command does not work

aaron@local bin % go get -u -d github.com/golang-migrate/migrate/cmd/migrate
go get: module github.com/golang-migrate/migrate@upgrade found (v3.5.4+incompatible), but does not contain package github.com/golang-migrate/migrate/cmd/migrate

Steps to Reproduce
Steps to reproduce the behavior:

1. My migrations look like '...'
2. I ran migrate with the following options '....'
3. See error

Expected Behavior
A clear and concise description of what you expected to happen.

Migrate Version
e.g. v3.4.0
Obtained by running: migrate -version

Loaded Source Drivers
e.g. s3, github, go-bindata, gcs, file
Obtained by running: migrate -help

Loaded Database Drivers
e.g. spanner, stub, clickhouse, cockroachdb, crdb-postgres, postgres, postgresql, pgx, redshift, cassandra, cockroach, mysql
Obtained by running: migrate -help

Go Version
e.g. go version go1.11 linux/amd64
Obtained by running: go version

Stacktrace
Please provide if available

Additional context
Add any other context about the problem here.