add swift flatbuffers

flatbuffers/Documentation.docc/Resources/code/swift/swift_code_3.swift

@@ -0,0 +1,7 @@
+import FlatBuffers
+import Foundation
+
+func run() {
+  // create a `FlatBufferBuilder`, which will be used to serialize objects
+  let builder = FlatBufferBuilder(initialSize: 1024)
+}