Add note about browsers

setup.md

@@ -46,7 +46,13 @@ Once you're done, you should only see an empty prompt like this:
 Wait for a few seconds to see that nothing else runs.
 
 Now look at your left side-panel. You should see a file named `notebook.ipynb`. Click on it.
-A new tab opens.
+A new tab with the notebook opens.
+
+> If the notebook does not show up after several minutes (and you only see the "blue progress bar" on top),
+> you might want to try a different browser. There have been reports about this happening with
+> Safari on OSX and with firefox private windows.
+{: .callout}
+
 Try to execute the first line to see that everything is set up properly.
 
 You're ready to go 🎉