[Error Message] Improved the error and added a link (#411)

unity-environment/Assets/ML-Agents/Scripts/CoreBrainInternal.cs

         serializedBrain.ApplyModifiedProperties();
 #endif
 #if !ENABLE_TENSORFLOW && UNITY_EDITOR
-        EditorGUILayout.HelpBox (@"You need to install the TensorflowSharp plugin in order to use the internal brain.", MessageType.Error);
+        EditorGUILayout.HelpBox (
+            "You need to install and enable the TensorflowSharp plugin in"+ 
+            "order to use the internal brain.", MessageType.Error);
+        if (GUILayout.Button("Show me how"))
+        {
+            Application.OpenURL("https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Getting-Started-with-Balance-Ball.md#setting-up-tensorflowsharp-support");
+        }
 #endif
     }