Add Getting started with Draftail extensions (#75)

https://thib.me/getting-started-with-draftail-extensions

A short "getting started" article which answers the basic questions.

README.md

@@ -185,6 +185,7 @@ Awesome Wagtail [![Awesome](https://cdn.rawgit.com/sindresorhus/awesome/d7305f38
 - [Python CMS Framework Review: Wagtail vs Django-CMS](https://blog.michaelyin.info/2017/06/19/python-cms-framework-review-wagtail-vs-django-cms/) - Talk about the difference between Django-CMS and Wagtail, the two most popular CMS framework in Python world.
 - [Deploying Wagtail In Production](https://vix.digital/insights/deploying-wagtail-production/)
 - [Setting Up Foundation Sass With Wagtail](https://vix.digital/insights/setting-foundation-sass-wagtail/)
+- [Getting started with Draftail extensions](https://thib.me/getting-started-with-draftail-extensions) – Do you want to write extensions for Draftail? This is a good place to start.
 
 ### Recipes