message: Mention that Xcode is a prereq
diff: @@ -21,6 +21,10 @@ Don't know what Artsy is? Check out [this overview](https://github.com/artsy/met
  Want to know more about Emission? Read the [mobile](http://artsy.github.io/blog/categories/mobile/) blog posts, or [Emission's](http://artsy.github.io/blog/categories/emission/) / [React Native's](http://artsy.github.io/blog/categories/reactnative/) specifically. Also check out [the map to Emission](docs/map_to_emission.md) to oriented yourself to the codebase.
  +### Prerequisites
  +
  +* Xcode
  +
  ### Installation
  _Automated install_:
  