Document how to add custom plugins to the item library

This task was: Allow a way for pure QML custom components to have an icon in QtC Designer "Component Library"

Currently we can create custom items, and as of 3.2-beta, they are shown in the panel. But they are all blank, they lack a recognizable icon and make it difficult to locate if not familiar. Just adding an icon would benefit as we start creating our own widget toolsets. Of course this icon is project-specific.

Maybe inside the qmldir file or a custom .qtcreator-extras.

This would prepare terrain for creating highly usable and reusable "UI-kits"