Add signac. (#186)

https://signac.io

Author: csadorf

_sections/30-projects.md

@@ -67,6 +67,7 @@ These projects pledge to drop Python 2 support in or before 2020.
 - [![](assets/angr.png)angr](http://angr.io/)
 - [![](assets/biobuilds.png)BioBuilds](https://www.biobuilds.org/)
 - [![](assets/freud.png)freud](https://github.com/glotzerlab/freud)
+- [![](assets/signac.png)signac](https://signac.io)
 
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