Docs: clarified usage of go get and go mod (#17637)

pkg/README.md

@@ -35,14 +35,17 @@ The Grafana project uses [Go modules](https://golang.org/cmd/go/#hdr-Modules__mo
 
 All dependencies are vendored in the `vendor/` directory.
 
+_Note:_ Since most developers of Grafana still use the `GOPATH` we need to specify `GO111MODULE=on` to make `go mod` and `got get` work as intended. If you have setup Grafana outside of the `GOPATH` on your machine you can skip `GO111MODULE=on` when running the commands below.
+
 To add or update a new dependency, use the `go get` command:
 
 ```bash
+# The GO111MODULE variable can be omitted when the code isn't located in GOPATH.
 # Pick the latest tagged release.
-go get example.com/some/module/pkg
+GO111MODULE=on go get example.com/some/module/pkg
 
 # Pick a specific version.
-go get example.com/some/module/pkg@vX.Y.Z
+GO111MODULE=on go get example.com/some/module/pkg@vX.Y.Z
 ```
 
 Tidy up the `go.mod` and `go.sum` files and copy the new/updated dependency to the `vendor/` directory: