Document how to set codefresh_pipeline spec_template to "AUTO-SELECT" branch

Ran into a use case that burned a bit of time figuring out and would like to update the docs to save someone else time!

To set up a pipeline so the YAML is automatically pulled from the triggering branch like so:

    spec_template {
      repo     = "${var.github_org}/${codefresh_project.application.name}"
      path     = "./codefresh/${var.pipeline_template}"
      location = "git"
      revision = ""
    }