Add todos for Entity Id and type. Add example usage of OpenAPI entity schema support in Playground

JSONAPI.playground/Pages/OpenAPI Documentation.xcplaygroundpage/Contents.swift

@@ -0,0 +1,13 @@
+//: [Previous](@previous)
+
+import Foundation
+import JSONAPI
+import JSONAPIOpenAPI
+
+// print Entity Schema
+let encoder = JSONEncoder()
+encoder.outputFormatting = .prettyPrinted
+
+let personSchemaData = try? encoder.encode(Person.openAPINode())
+
+print(personSchemaData.map { String(data: $0, encoding: .utf8)! } ?? "Schema Construction Failed")