Added ECS docs

docs/system/system.md

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+# System
+### Contents
+[About](#About)
+
+## About
+The system runs operations that must be performed on its assoiciated components.
+
+Typically, this is expected to be run every frame, though this is not a strict requirement.
+
+Examples of the `system` include the `health_system`; which reads the value of the entitie's `health` component and ensures h > 0.
+``` java
+void health_system(){
+    for (i = 0; i < registeredComponents.size(); i++){
+        if (registeredComponents[i].health <= 0){
+            registeredComponents[i].dead = true;
+            if (registeredComponents[i].isPlayer){
+                GameOver();
+            }
+        }
+    }
+}
+```
+
+