Update Ball3DDecision.cs (#218)

I think it is important to add a simple heuristic example to demonstrate how to implement a Decision

unity-environment/Assets/ML-Agents/Examples/3DBall/Scripts/Ball3DDecision.cs

     {
         if (gameObject.GetComponent<Brain>().brainParameters.actionSpaceType == StateType.continuous)
         {
-            return new float[4]{ 0f, 0f, 0f, 0.0f };
+            List<float> ret = new List<float>();
+            if (state[2] < 0 || state[5] < 0)
+            {
+                ret.Add(state[5]);
+            }
+            else
+            {
+                ret.Add(state[5]);
+            }
+            if (state[3] < 0 || state[7] < 0)
+            {
+                ret.Add(-state[7]);
+            }
+            else
+            {
+                ret.Add(-state[7]);
+            }
+            return ret.ToArray();
 
         }
         else