Add Zettlr (#1093)

README.md

@@ -148,6 +148,7 @@ Any comments, suggestions? [Let us know!](https://github.com/jaywcjlove/awesome-
 * [TextNut](http://www.textnutwriter.com/) - Rich-format editor featuring Markdown export and Markdown syntax hints. ![Freeware][Freeware Icon]
 * [Typora](http://www.typora.io/) - Truly minimal Markdown editor featuring seamless live preview. ![Freeware][Freeware Icon]
 * [Ulysses](https://www.ulyssesapp.com/features/) - The Ultimate Writing App for Mac, iPad and iPhone.
+* [Zettlr](https://www.zettlr.com/) - A markdown editor for the 21st century. [![Open-Source Software][OSS Icon]](https://github.com/Zettlr/Zettlr) ![Freeware][Freeware Icon]
 
 #### Note-taking