Added info about the build overlay branches to README

README.md

@@ -46,3 +46,8 @@ if rhs != nil { lhs = rhs }
 // or
 if let rhs = rhs { lhs = rhs }
 ```
+
+
+## Build Overlays
+
+The master branch is Swift 5.x, 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-4.2](https://github.com/capnslipp/NilCoalescingAssignmentOperators/tree/swift-4.2), [swift-4](https://github.com/capnslipp/NilCoalescingAssignmentOperators/tree/swift-4), and [swift-3](https://github.com/capnslipp/NilCoalescingAssignmentOperators/tree/swift-3) branches.  _(Note: I don't check that these are built 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.)_