Add BioBuilds to list of participating projects

BioBuilds will no longer include Python 2.x packages in our standard
environment(s) by the 2019.12 release. (Existing Python 2.7 packages
will still available through our conda channel; however, they will not
be included as part of a standard BioBuilds install.)

Closes biobuilds/biobuilds#18

Author: Cheng H. Lee

_sections/30-projects.md

@@ -60,6 +60,7 @@ These projects pledge to drop Python 2 support in or before 2020.
 - [![](assets/pymeasure.png)PyMeasure](https://github.com/ralph-group/pymeasure)
 - [![](assets/chaquopy.png)Chaquopy](https://chaquo.com/chaquopy/)
 - [![](assets/angr.png)angr](http://angr.io/)
+- [![](assets/biobuilds.png)angr](https://www.biobuilds.org/)
 
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