explain the process.platform check

docs/tutorial/quick-start.md

@@ -82,6 +82,8 @@ var mainWindow = null;
 
 // Quit when all windows are closed.
 app.on('window-all-closed', function() {
+  // On OSX it is common for applications and their menu bar 
+  // to stay active until the user quits explicitly with Cmd + Q
   if (process.platform != 'darwin') {
     app.quit();
   }