Clarify docker/go package
Clarify docker/go package

This package is used by Notary. Add a comment to
the vendor.conf file to explain what it contains
and what it's used for.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

cpuguy83/cli,dnephin/cli,thaJeztah/cli,docker/cli,Zebrilee/cli,ashfall/cli,thaJeztah/cli,allencloud/cli,Zebrilee/cli,redpanda/cli,cpuguy83/cli,allencloud/cli,riyazdf/cli,docker/cli,dnephin/cli,riyazdf/cli,redpanda/cli,ashfall/cli