Update playground and README for computed attributes.

JSONAPI.playground/Pages/Usage.xcplaygroundpage/Contents.swift

@@ -44,6 +44,7 @@ let housesFromData = peopleResponse.body.includes?[House.self]
 
 if case let .data(bodyData) = peopleResponse.body {
 	print(bodyData)
+	print("first person's name: \(bodyData.primary.values[0][\.fullName])")
 } else {
 	print("no body data")
 }

JSONAPI.playground/Sources/Entities.swift

@@ -35,6 +35,10 @@ public enum PersonDescription: EntityDescription {
 		public let name: Attribute<[String]>
 		public let favoriteColor: Attribute<String>
 
+		public var fullName: Attribute<String> {
+			return name.map { $0.joined(separator: " ") }
+		}
+
 		public init(name: Attribute<[String]>, favoriteColor: Attribute<String>) {
 			self.name = name
 			self.favoriteColor = favoriteColor