Document custom types, catalogs behavior

[Ivshti]

Document a few important characteristics of the addon system

1. catalogs is completely independent of resources and types; you don't have to define the catalogs resource, or you don't have to define a type in types in order for you to define a catalog with that type; docs need to be updated to make this clear
2. this means that, in order to show all types that an add-on supports, you have to concat types from catalogs, types and even resources (since resources can be defined with an object that further specifies types, or full notation)
3. or alternatively, display as this addon provides catalogs for {allTypesFromCatalogs.join(', ')}, and {allResourceNames.join(', ')} for {types.concat(typesFromResources).join(', ')}; or even better, one line of all the catalog types, and then one line for each resource (e.g. "Subtitles for movies, series") where the line would take either resource.types (if using full notation) or manifest.types if using short notation