Add Entity validation via a function in JSONAPITestLib.

JSONAPI.playground/Pages/Test Library.xcplaygroundpage/Contents.swift

@@ -14,3 +14,11 @@ Please enjoy these examples, but allow me the forced casting and the lack of err
 // The JSONAPITestLib provides literal expressibility for key types to
 // make creating tests easier
 let dog = Dog(id: "1234", attributes: Dog.Attributes(name: "Buddy"), relationships: Dog.Relationships(owner: nil))
+
+// MARK: - JSON API structure checking
+// The JSONAPITestLib provides a `check` function for each Entity type
+// that uses reflection to catch mistakes that are not forbidden by
+// Swift's type system but will result in unexpected results when
+// encoding/decoding. It is a good idea to add a `check` to each of
+// your unit tests that create Entities.
+try Dog.check(dog)