title: How to add files to Xcode project? (writing Unity plugin)
body: <p>I bet my problem is very basic, so please excuse me as I'm not that experienced with Xcode, since I work in Unity and use ready plugins for our iOS projects, so not really needed to go deep in Xcode until now.</p>
  
  <p>I want to write a very basic custom plugin, but cannot get Xcode to accept it. I followed the tutorial found on the Unity page, and added my .h and .m files to Assets / Plugins / iOS, where (as the tutorial states) they will be merged automatically into Xcode.</p>
  
  <p>After building, Xcode throws two errors:
  *"Undefined symbols for architecture armv7:
    "__XY", referenced from:
        __Z19RegisterMonoModulesv in RegisterMonoModules.o"*
  And the same for armv6.</p>
  
  <p>As I read in similar issues here, this is because the files are not added to the Xcode project, despite the fact, that they should be added automatically. In Xcode, I can see both my files in the Project Navigator. I also see my .mm file in the Build Phases list.</p>
  
  <p>What's the problem?</p>