Add connection timeout issue

Author: awjuliani

docs/Limitations-&-Common-Issues.md

@@ -12,7 +12,7 @@ When running a Unity Environment on macOS using Metal rendering, the application
 
 ## Python API
 
-### Unity Environment Permission Error
+### Environment Permission Error
 
 If you directly import your Unity environment without building it in the editor, you might need to give it additionnal permissions to execute it. 
 
@@ -26,6 +26,12 @@ or on Linux:
 
 On Windows, you can find instructions [here](https://technet.microsoft.com/en-us/library/cc754344(v=ws.11).aspx).
 
+### Environment Connection Timeout
+
+If you are able to launch the environment from `UnityEnvironment` but then recieve a timeout error, there may be a number of possible causes.
+ * _Cause_: There may be no Brains in your environment which are set to `External`.  In this case, the environment will not attempt to communicate with python. _Solution_: Set the train you wish to externally control through the Python API to `External` from the Unity Editor, and rebuild the environment.
+ * _Cause_: On OSX, the firewall may be preventing communication with the environment. _Solution_: Add the built environment binary to the list of exceptions on the firewall by following instructions [here](https://support.apple.com/en-us/HT201642). 
+
 ### Filename not found
 
 If you receive a file-not-found error while attempting to launch an environment, ensure that the environment files are in the root repository directory. For example, if there is a sub-folder containing the environment files, those files should be removed from the sub-folder and moved to the root.