Document the `.swift-version` file warning and working around it.
Document the `.swift-version` file warning and working around it.

apple/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf