Added usage sample for PriorityQueue

Author: eleev

Data Structures.playground/Pages/PriorityQueue.xcplaygroundpage/Contents.swift

@@ -0,0 +1,12 @@
+//: [Previous](@previous)
+
+import Foundation
+
+//: Usage: 
+
+var queue = PriorityQueue<Int>(elements: [2, 1, 4, 3, 5], order: >)
+queue.enqueue(9)
+
+print(#function + " queue: " , queue)
+
+//: [Next](@next)

Data Structures.playground/contents.xcplayground

@@ -9,5 +9,6 @@
         <page name='Tree'/>
         <page name='Graph'/>
         <page name='Heap'/>
+        <page name='PriorityQueue'/>
     </pages>
 </playground>