Describe in README why all relevant decisions were made

Things like 
- Why Xcodegen?
- Why .xxx-version files?
- Why have a Core framework?
- Why SwiftGen?
- Why check that files aren't changed after running rebuild scripts (e.g. `$ make swiftgen`)?
- Why use a layout framework?
- Why use UIStackView? (mention potential performance tradeoffs)
etc.