Add some hopefully useful documentation for people creating new interfaces

The generated classes don't really have documentation for the generated
methods. This hopes to provide some context.

src/wayland/server/DESIGN.md

+# History
+
+We started out with one method of generting classes. We then ported to a new approach of using QtWaylandScanner to reduce a lot of boiler plate.
+
+New classes should use the new approach.
+
+# New Approach
+
+A public facing PIMPL class which should inherit from QObject.
+A private class that should inherit from QtWaylandServer::interface_name which is auto-generated. This will manage individual resources, handle callbacks and QString conversions.
+
+Class Names should map to the interface name in UpperCamelCase.
+Where a V1 exists in the interface name, this should be mirrored in the file and class name.
+
+An implementation should handle all versions of a given interface, but not different interfaces which represent different versions.
+
+(i.e zxdg_output_manager_v1 versions 1 2 and 3 would be wrapped in one class XdgOutputManagerV1Interface. A zxdg_output_manager_v2 protocol would be exposed as a new public class XdgOutputManagerV2Interface)
+
+# Implementations
+
+There are 3 modes of operation happening within the exported classes of KWaylandServer
+
+The generated classes can behave in all these modes, it is up to our implementation to use the correct methods.
+
+## Globals
+e.g BlurManager
+
+This represents an object listed by the Display class.
+Use the interface_name::(wl_display*, int version) constructor within the private class to create an instance.
+
+
+## Server-managed multicasting resources
+e.g XdgOutput
+
+This is where one QObject represents multiple Resources.
+
+Use the method
+```cpp
+QtWaylandServer::interface_name::add(client, id, version)
+```
+
+to create a a new Resource instance managed by this object.
+
+Use the event method with the wl_resource* overload to send events.
+
+```cpp
+for (auto resource : resourceMap())
+{
+    send_some_method(resource->handle, arg1, arg2);
+}
+```
+
+methods to send requests to all clients.
+
+## Client-owned Resources:
+
+e.g BlurInterface
+
+This is where one instance of our public class represents a single resource. Typically the lifespan of the exported class matches our resource.
+
+In the private class use the QtWaylandServer::interface_name(wl_resource*) constructor to create a wrapper bound to a specific resource.
+
+Use
+```cpp
+send_some_method(arg1, args2)
+```
+methods of the privateClass to send events to the resource set in the constructor
+
+
+## Other hooks
+
+`_bind_resource` is called whenever a resource is bound. This exists for all generated classes in all the operating modes
+
+`_destroy_resource` is a hook called whenever a resource has been unbound.
+Note one should not call wl_resource_destroy in this hook.
+
+## Resource destructors
+
+destructors (tagged with type="destructor" in the XML) are not handled specially in QtWayland it is up to the class implementation to implement the event handler and call 
+```cpp
+wl_resource_destroy(resource->handle)
+```
+in the relevant method
+