Added info to README about build overlays

README.md

@@ -1,3 +1,9 @@
 # SCNViewController
 
 SCNViewController is a Swift micro-library that provides a configurable/reusable `SCNViewController` class— a `UIViewController` subclass with a SceneKit `SCNView`.
+
+
+
+## Build Overlays
+
+The master branch is Swift ≥5.5, and build overlays (the minimal changeset to the Package.swift, xcodeproj, and other build files) of the current library version are available on the [swift-5.0](https://github.com/capnslipp/SCNViewController/tree/swift-5.0), [swift-4.2](https://github.com/capnslipp/SCNViewController/tree/swift-4.2), [swift-4](https://github.com/capnslipp/SCNViewController/tree/swift-4), and [swift-3](https://github.com/capnslipp/SCNViewController/tree/swift-3) branches.  _(Note: I don't check these builds as often as I used to when Swift 4.2 or 4.0 were the latest versions, but their changes haved worked and I've merged new library versions into them since then.)_