Add a note about needed polyfills

Fixes #125.

readme.md

@@ -104,7 +104,8 @@ Furthermore, I try to keep this project as transparently stable and updated as f
 The official recommendation for running `common-tags` is as follows:
 
 - [Node.js](https://nodejs.org/en/download/) v5.0.0 or higher
-- In order to use `common-tags`, your environment will also need to support ES2015 tagged templates ([pssst… check Babel out](http://babeljs.io)).
+- In order to use `common-tags`, your environment will also need to support ES2015 tagged templates ([pssst… check Babel out](http://babeljs.io))
+- You might also want to [polyfill some features](https://github.com/zloirock/core-js) if you plan on supporting older browsers: `Array.prototype.includes`
 
 It might work with below versions of Node, but this is not a guarantee.