make note about template feature

src/System.cs

@@ -26,6 +26,7 @@ namespace MoonTools.ECS
 
 		protected void Set<TComponent>(in Template template, in TComponent component) where TComponent : unmanaged => World.Set<TComponent>(template, component);
 
+		// This feature is EXPERIMENTAL. USe at your own risk!!
 		protected Entity Instantiate(in Template template) => World.Instantiate(template);
 
 		protected ReadOnlySpan<TMessage> ReadMessages<TMessage>() where TMessage : unmanaged

src/Template.cs

@@ -1,5 +1,6 @@
 namespace MoonTools.ECS
 {
+	// This feature is EXPERIMENTAL. Use at your own risk!!
 	public struct Template
 	{
 		public int ID { get; }