Merge pull request #1074 from SaintPeter/staging

Add video link to Symmetric Differences Bonfire

seed/bonfireMDNlinks.js

@@ -13,6 +13,7 @@ var links =
   "Smallest Common Multiple": "https://www.mathsisfun.com/least-common-multiple.html",
   "Permutations": "https://www.mathsisfun.com/combinatorics/combinations-permutations.html",
   "HTML Entities": "http://dev.w3.org/html5/html-author/charref",
+  "Symmetric Difference": "https://www.youtube.com/watch?v=PxffSUQRkG4",
 
 	// ========= GLOBAL OBJECTS
 	"Global Array Object" : "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array",

seed/challenges/advanced-bonfires.json

@@ -87,7 +87,8 @@
         "assert.deepEqual(sym([1, 1]), [1], 'should return an array of unique values');"
       ],
       "MDNlinks": [
-        "Array.reduce()"
+        "Array.reduce()",
+        "Symmetric Difference"
       ],
       "challengeType": 5,
       "nameCn": "",