-some documentation changes

-added Component onCreate callback which is called after allocating entity from template

source/ecs/system.d

@@ -1,3 +1,6 @@
+/************************************************************************************************************************
+*System module.
+*/
 module ecs.system;
 
 import ecs.entity;
@@ -5,6 +8,13 @@ import ecs.manager;
 
 /************************************************************************************************************************
 *System contain data required to proper glue EntityManager with Systems.
+*System callbacks:
+*<br/>-void onEnable()
+*<br/>-void onDisable();
+*<br/>-bool onBegin();
+*<br/>-void onEnd();
+*<br/>-void onCreate()
+*<br/>-void onDestroy();
 */
 struct System
 {