README: Tweak description

Author: mathiasbynens

README.md

@@ -1,6 +1,6 @@
 # ES6 `String.prototype.codePointAt` polyfill [![Build status](https://travis-ci.org/mathiasbynens/String.prototype.codePointAt.png?branch=master)](https://travis-ci.org/mathiasbynens/String.prototype.codePointAt)
 
-A robust & optimized polyfill for [the `String.prototype.codePointAt` method in ECMAScript 6](http://people.mozilla.org/~jorendorff/es6-draft.html#sec-21.1.3.3).
+A robust & optimized ES3-compatible polyfill for [the `String.prototype.codePointAt` method in ECMAScript 6](http://people.mozilla.org/~jorendorff/es6-draft.html#sec-21.1.3.3).
 
 Other polyfills for `String.prototype.codePointAt` are available: