Add Zulip.

Author: rht

_sections/30-projects.md

@@ -30,6 +30,7 @@ These projects pledge to drop Python 2 support in or before 2020.
 - [![](assets/rdkit.png)RDKit](https://github.com/rdkit/rdkit)
 - [![](assets/kivy.png)Kivy](https://kivy.org/)
 - [![](assets/biopython.png)Biopython](http://biopython.org/)
+- [![](assets/zulip.png)Zulip](https://zulip.org)
 
 &nbsp; <!--break separating project with image from without -->