make a troubleshooting note about preparing safari directory [ci skip]

docs/en/troubleshooting.md

@@ -60,7 +60,8 @@ own items in the SDK manager. Make sure you install the build-tools and platform
   of Instruments. Usually a box will pop up that you enter your password into
   . If you're running Appium from source, you can simply run `sudo grunt authorize`
   from the main repo to avoid getting this popup. If you're running from npm,
-  run `sudo authorize_ios` instead.
+  run `sudo authorize_ios` instead. You need to do this every time you install
+  a new version of Xcode, as well.
 * If you see `iOS Simulator failed to install the application.` and the
   paths are correct, try restarting the computer.
 
@@ -118,3 +119,4 @@ output when it's run in verbose mode so that we can diagnose what's going on.
 |Running reset.sh|xcodebuild: error: SDK "iphonesimulator6.1" cannot be located|Install the iPhone 6.1 SDK _or_ build the test apps with a separate SDK, e.g., `grunt buildApp:UICatalog:iphonesimulator5.1`|
 |Running reset.sh|Warning: Task "setGitRev" not found. Use --force to continue.|Update the submodules with `git submodule update --init` and run `reset.sh` again|
 |Running ios test|`[INST STDERR] posix spawn failure; aborting launch`|Your app is not compiled correctly for the simulator or device.|
+|Running mobile safari test|`error: Could not prepare mobile safari with version '7.1'`|You probably need to run the authorize script again to make the iOS SDK files writeable. E.g., `sudo authorize_ios`|