Add guide for installing a specific version of migrate using the Go…

… toolchain

cli/README.md

@@ -27,10 +27,20 @@ $ apt-get install -y migrate
 
 #### With Go toolchain
 
+##### Unversioned
 ```
 $ go get -tags 'postgres' -u github.com/golang-migrate/migrate/cmd/migrate
 ```
 
+##### Versioned
+
+```
+$ go get -u -d github.com/golang-migrate/migrate/cmd/migrate
+$ cd $GOPATH/src/github.com/golang-migrate/migrate/cmd/migrate
+$ git checkout $TAG  # e.g. v4.1.0
+$ go build -tags 'postgres' -ldflags="-X main.Version=$(git describe --tags)" -o $GOPATH/bin/migrate github.com/golang-migrate/migrate/cmd/migrate
+```
+
 ##### Notes:
 1. Requires a version of Go that [supports modules](https://golang.org/cmd/go/#hdr-Preliminary_module_support). e.g. Go 1.11+
 1. This example builds the cli which will only work with postgres.  In order