add troubleshooting to README

Author: ErinWeisbart

README.md

@@ -12,3 +12,17 @@ A module may be in CellProfiler-plugins instead of CellProfiler itself because:
 - it has been contributed by a community member
 
 Please see our [CellProfiler-plugins documentation](https://plugins.cellprofiler.org) for more information about installation, currently supported plugins, and how to contribute.
+
+## Troubleshooting
+
+If CellProfiler won't open after setting the CellProfiler Plugins folder (and it returns `error: no commands supplied` in the terminal), it is likely because you have set the CellProfiler Plugins folder to the parent folder of the plugins repository (`CellProfiler-plugins`), not the folder that contains plugins (`CellProfiler-plugins/active_plugins`). 
+
+In order to get CellProfiler to open, remove `setup.py` from the `CellProfiler-plugins` folder. 
+Open CellProfiler.
+Change the CellProfiler Plugins path to the correct path and close CellProfiler. 
+Return setup.py to the parent folder.  
+OR  
+Alternatively, you can edit the `PluginDirectory` line of your config to the correct path and then reload CellProfiler.
+On Mac, you can find the config at `/Users/{username}/Library/Preferences/CellProfilerLocal.cfg`
+
+For other troubleshooting information, please see the [Troubleshooting](https://plugins.cellprofiler.org/troubleshooting.html) page of our documentation.