Adding some documentation about how fetching embedded resources works

lib/waterwheel.js

@@ -147,8 +147,12 @@ module.exports = class Waterwheel extends Request {
       });
     });
 
+    // Create a Set from the possibly-duplicate links array.
+    // Get an array from that set.
     links = Array.from(new Set(links));
 
+    // Promise.all accepts an array of promises to resolve. The first item
+    // in this array is the original entity, adjacent to the embedded ones.
     return Promise.all([Promise.resolve(entityJSON)].concat(links.map(link => this.issueRequest(methods.get, link))));
   }