Add docs on how to pin gitlabktl to a specific version
Problem to solve
As a DevOps Engineer, in order to make sure my builds will never fail unexpectedly, I would like to pin the gitlabktl version of my builds.
Document how to pin
gitlabktl to a specific version.
This is an issue to deprecate Knative 0.5 support in a nice way, related to gitlabktl#55