Improve container-test README
<!-- Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root. -->
# Container-test
-Convenience dependency for users developing OSGi bundles for JDisc.
-Add this maven artifact as a **test** scope dependency in your pom.xml,
-and it will transitively pull in all dependencies needed to unit test
-JDisc components with the `application` test tool.
+Convenience dependency for testing plugin components for the Jdisc container
+with the `application` test tool. This artifact should contain all libraries
+that are used internally by Vespa. Add this maven artifact as a **test** scope
+dependency in your pom.xml to transitively pull in all dependencies needed to unit test
+JDisc components.
+
+This should always be used in conjunction with a `provided` scoped dependency
+on the `container` artifact, which contains all internal and 3rd party artifacts
+that are exposed in Vespa's public APIs. (Internal Vespa developers should instead
+use the `container-dev` artifact.)
