This is a command line tool (disguised as an app) for generating code for a project. The use is as follows:
CodeGen.app/Contents/MacOS/CodeGen -target <source-root> -templates <template-dir> [-clean]
The code will scan the given source-root directory for any .swift files using SourceKitten, and generate code based on templates written using the Stencil syntax. Generated code is then written to Autogen/Autogen.swift.
Examples of valid templates can be found in the test fixtures directory
To add a code generation step into your project, you will need to hit Product -> Archive in Xcode, and then export it as a Mac App.
Once done, copy the .app file into your project, and follow the steps in this article.
The first build will fail, as the file Autogen.swift won't have been added to your target yet, as it didn't exist - just add that file to your target, and you should be good to go.
Also note that GenKit generates the file in a diff-friendly way, so you can check in the Autogen.swift file if you wish, to help smooth over this step.