added entity script example

docs/Scripting/entity_scripts.rst

@@ -1,2 +1,31 @@
 Entity scripts
-==============
+==============
+
+Entity scripts are Wren scripts that are attached to an entity using the WrenScript component.
+
+After attaching the WrenScript component on an entity, you can select the script file directly.
+In addition to pointing to the script file, you need to specify which class in the script is considered as the
+entity script.
+
+The class chosen must inherite from ScriptableEntity.
+
+Here is an example of a barebone entity scripts::
+
+    import "Scripts/ScriptableEntity" for ScriptableEntity
+
+    class TestScript is ScriptableEntity {
+        construct new() {
+        }
+
+        init() {
+
+        }
+
+        update(ts) {
+           
+        }
+
+        exit() {
+            
+        }
+    }