code: public synchronized void add(T key, Resource resource)
      {
          Check.notNull(key);
          Check.notNull(resource);
  
          if (started.get())
          {
              throw new LionEngineException(ERROR_STARTED);
          }
          resources.put(key, resource);
      }
docs: Add a resource to load. Must be called before {@link #start()}.
  
  @param key The associated key (must not be <code>null</code>).
  @param resource The resource to load (must not be <code>null</code>).
  @throws LionEngineException If invalid arguments or loader has already been started.