Added simple navigation, some more implementation details

docs/system/system.md

@@ -1,7 +1,13 @@
+*[JavaECS](../../README.md) / [docs](../overview.md) / system*
 # System
+
 ### Contents
 [About](#About)
 
+[Implementation](#Implementation)
+
+[Examples](#Examples)
+
 ## About
 The system runs operations that must be performed on its assoiciated components.
 
@@ -20,5 +26,22 @@ void health_system(){
     }
 }
 ```
+## Implementation
+
+The systems are managed by the following code:
+
+``` java
+class SystemManager{
+    ECS baseECS;
+    int systemIndex;
+
+    public SystemManager(ECS baseECS){
+        this.baseECS = baseECS;
+        systemIndex = 0;
+    }
+}
+```
+
+## Examples