Clarify namespacing for addons invoking their own components.

The Core Team decided that it would be most clear if addons invoked 
helpers and components with the same namespacing to be used by
consumers of the addon. Without a namespace, components could be 
single non-dasherized words, which is verboten. Furthermore, this
decision makes templates more portable - it will be easier for an 
addon to adopt a component from a consumer, for instance.

text/0000-module-unification.md

@@ -538,6 +538,12 @@ and a top-level `main` component declared in `src/ui/components/main.js`. An
 app could invoke this component in a template as `{{power-select::main}}` or
 more simply as `{{power-select}}`.
 
+Addons should use the same namespacing that will be used by consuming apps when
+invoking their own components and helpers from templates. For instance, if the
+`ember-power-select` addon has a `date-picker` component that invokes multiple
+`main` components, it should also invoke them in a template as
+`{{power-select::main}}` or more simply as `{{power-select}}`.
+
 ### Module Resolutions
 
 Module resolution rules must account for the following: