docs(readme): add introduction

README.md

@@ -1 +1,14 @@
-# equal-array
+# equal-array
+
+equal-array compares arrays by values instead of references.
+
+## Why ?
+
+It is mainly useful with an es6 Map or Set, where arrays, used as keys, are compared by references. For example :
+```js
+const map = new Map();
+map.set([1, 2 ,3], 'a');
+map.set([1, 2 ,3], 'b'); // the map will consider [1, 2 ,3] as a new key
+map.size(); // returns 2 - with equal-array: returns 1
+map.has([1, 2, 3]); // returns false - with equal-array: returns true
+```