Adds Psi4 (python3statement#136)

* Add Psi4 to list

* Add Psi4 logo

* Add files via upload

* Update 30-projects.md

* Delete psi4square.gif

Author: loriab

_sections/30-projects.md

@@ -39,6 +39,7 @@ These projects pledge to drop Python 2 support in or before 2020.
 - [![](assets/fenics.png)FEniCS](http://www.fenicsproject.org)
 - [![](assets/rdkit.png)RDKit](https://github.com/rdkit/rdkit)
 - [![](assets/metpy.png)MetPy](https://unidata.github.io/MetPy)
+- [![](assets/psi4square.png)Psi4](http://psicode.org/)
 - [![](assets/openquake.png)OpenQuake engine](https://github.com/gem/oq-engine)
 - [![](assets/yt.png)yt](http://yt-project.org)
 - [![](assets/cmd2.png)cmd2](https://github.com/python-cmd2/cmd2)