Actually bring back IE11+ compability notice on docs since SauceLabs …

…tests proved IE tests to fail

docs/index.md

@@ -1,8 +1,18 @@
 # Get started
 
-*ProgressBar.js* is lightweight, MIT licensed and supports all major browsers including **IE9+**.
+*ProgressBar.js* is lightweight, MIT licensed and supports all major browsers including **IE11+** when using a polyfill.
 See complete examples in [full examples](#full-examples) section.
 
+### IE 11 compatibility
+
+You can include ES6 polyfills by adding to your index.html:
+
+```
+<script crossorigin="anonymous" src="https://polyfill.io/v3/polyfill.min.js"></script>
+```
+
+See https://github.com/jeremyckahn/shifty#ie-compatibility and https://polyfill.io/v3/url-builder/.
+
 #### {install} Installing
 
 * Using bower `bower install progressbar.js`