Add CONTRIBUTING.md (#90)

* Add CONTRIBUTING.md to note bin/carthage command

* Update CONTRIBUTING to reflect removal of Carthage

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+# Contributing to turbo-ios
+
+To set up your development environment:
+
+1. Clone the repo
+1. Open the directory in Xcode to install Swift packages
+
+To run the test suite:
+
+1. Open the directory in Xcode
+1. Click Product → Test or <kbd>⌘</kbd>+<kbd>U</kbd>

README.md

@@ -46,6 +46,8 @@ You can also integrate the framework manually if your prefer, such as by adding
 
 ## Contributing
 
+Read the [contributing guide](/CONTRIBUTING.md) to learn how to set up your development environment.
+
 Turbo iOS is open-source software, freely distributable under the terms of an [MIT-style license](LICENSE). The [source code is hosted on GitHub](https://github.com/hotwired/turbo-ios).
 Development is sponsored by [37signals](https://37signals.com/).