Add explanatory comment to empty BUILD file(s)
Add explanatory comment to empty BUILD file(s)

Change-Id: Ifc4e5146672ffa35ba45882cafaae82bf9ea5751

GerritCodeReview/plugins_serviceuser,GerritCodeReview/plugins_serviceuser,GerritCodeReview/plugins_serviceuser,GerritCodeReview/plugins_serviceuser,GerritCodeReview/plugins_serviceuser