Need better instructions on go mod vendor before creating the release

[garethjevans]

Currently, we have set GOPROXY=off in the packaging scripts for the golang packages which stop go build from trying to access the internet when building each binary, but... this required go mod download && go mod vendor to be run before creating the bosh release. This will be a manual step that needs to be run before we have a pipeline that creates a release automatically.

We need to document this somewhere.

[KevinJCross]

we should have make target make release to generate a new release. This should be added to our documentation.

[silvestre]

We already document that a call to scripts/update is necessary: https://github.com/cloudfoundry/app-autoscaler-release/blob/061aeaa7e97a55e813cc9b3dcc8453b4542e8905/README.md#bosh-lite-deployment

We will however check how to make this clearer in the future.