Add article about document previews in Wagtail (#39)

http://filepreviews.io/blog/2017/04/20/adding-document-previews-to-wagtail/

README.md

@@ -164,6 +164,7 @@ Awesome Wagtail [![Awesome](https://cdn.rawgit.com/sindresorhus/awesome/d7305f38
 - [Extending The Functionality of Email Forms in Wagtail](https://posts-by.lb.ee/dev-wagtail-extending-the-functionality-of-email-forms-232c8469ac97)
 - [Wagtail: 2 Steps for Adding Pages Outside of the CMS](https://www.caktusgroup.com/blog/2016/02/15/wagtail-2-steps-adding-pages-outside-cms/)
 - [Code blocks for Wagtail using Pygments](https://jordijoan.me/code-blocks-wagtail-using-pygments/)
+- [Adding document previews to Wagtail CMS](http://filepreviews.io/blog/2017/04/20/adding-document-previews-to-wagtail/)
 
 ### Recipes