Describe in README why all relevant decisions were made

[n8chur]

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.