Document new docker.clean command

CHANGELOG.adoc

@@ -16,6 +16,7 @@ This project adheres to http://semver.org/[Semantic Versioning].
 
 * Add a Patternlab command to build the static style guide: `fab patternlab.build`.
 * Add a Git command to verify repositories states and warn user if changes might be lost.
+* Add a docker.clean command to clean docker's workspace (removes container and image).
 
 == 2.0.0 - 2016/05/09
 

docker.py

@@ -225,7 +225,7 @@ def image_remove():
 @roles('local')
 def clean():
     """
-    Clean docker workspace
+    Clean docker workspace (removes container & images)
     """
     if (confirm(
             red('This will stop, remove container and delete docker image ' +