Add note to file radar for Xcode Integration

Mention filing a radar that's a duplicate to an existing radar bug
(26152293) to help promote this feature for Xcode.

Addresses concerns in Quick#667.

Documentation/en-us/SettingUpYourXcodeProject.md

@@ -30,6 +30,7 @@ class MyClassTests: XCTestCase {
 ```
 
 > Quick integration in the Xcode Test Navigator suffers from some limitations (open [issue](https://github.com/Quick/Quick/issues/219)). Quick tests will not show up in the navigator until they've been run, repeat runs tend to reset the list in unpredictable ways and the tests cannot be run from the gutter next to the source code.
+> Please file a radar to Apple and mention this as a duplicate to [rdar://26152293](http://openradar.appspot.com/radar?id=4974047628623872) to promote this feature request for Apple Engineers.
 
 > Some developers advocate adding Swift source files to your test target.
 However, this leads to [subtle, hard-to-diagnose