Add comment about how it works (#2)

Co-authored-by: Sindre Sorhus <[email protected]>

index.js

@@ -1,3 +1,8 @@
+/*
+How it works:
+`this._head` is an instance of `Node` which keeps track of its current value and nests another instance of `Node` that keeps the value that comes after it. When a value is provided to `.enqueue()`, the code needs to iterate through `this._head`, going deeper and deeper to find the last value. However, iterating through every single item is slow. This problem is solved by saving a reference to the last value as `this._tail` so that it can reference it to add a new value.
+*/
+
 class Node {
 	/// value;
 	/// next;