PDF exporting also works with Chromium

For those who can't install Google Chrome (e.g. work network policies) but can use Chromium, Chromium does work for creating the pdf. I updated the readme accordingly. Thanks!

README.md

@@ -656,7 +656,7 @@ Reveal.initialize({
 
 ## PDF Export
 
-Presentations can be exported to PDF via a special print stylesheet. This feature requires that you use [Google Chrome](http://google.com/chrome).
+Presentations can be exported to PDF via a special print stylesheet. This feature requires that you use [Google Chrome](http://google.com/chrome) or [Chromium](https://www.chromium.org/Home).
 Here's an example of an exported presentation that's been uploaded to SlideShare: http://www.slideshare.net/hakimel/revealjs-300.
 
 1. Open your presentation with `print-pdf` included anywhere in the query string. This triggers the default index HTML to load the PDF print stylesheet ([css/print/pdf.css](https://github.com/hakimel/reveal.js/blob/master/css/print/pdf.css)). You can test this with [lab.hakim.se/reveal-js?print-pdf](http://lab.hakim.se/reveal-js?print-pdf).