Fix lightGBM macOS bug

[zyteka]

As described in #428 , users may encounter a bug in tutorials caused by lightgbm (related to this: https://stackoverflow.com/questions/50889162/lightgbm-image-not-found-after-mac-install). #436 addresses this for now by adding a troubleshooting guide. If lightgbm doesn't offer a fix soon, we should come up with a more permanent fix.