adding Zenodo reference

README.md

@@ -1,6 +1,9 @@
 [![CI-test](https://github.com/PrincetonUniversity/Vatic/actions/workflows/test.yml/badge.svg)](
 https://github.com/PrincetonUniversity/Vatic/actions/workflows/test.yml)
 
+[![DOI](https://zenodo.org/badge/406584271.svg)](https://zenodo.org/badge/latestdoi/406584271)
+
+
 # Vatic #
 
 Vatic is a Python package for running simulations of a power grid using the
@@ -26,6 +29,13 @@ cd Vatic
 pip install .
 ```
 
+While this will install Vatic's Python package dependencies, you will need to also choose and install a MILP solver
+with Python support. A good free option is [Cbc](https://github.com/coin-or/Cbc); we also recommend considering
+obtaining a license for [Gurobi](https://www.gurobi.com/), which allows for much faster simulation.
+
+
+### Installing grid datasets ###
+
 The Vatic repository includes the [May 2021 version of the Texas-7k grid dataset](
 https://electricgrids.engr.tamu.edu/electric-grid-test-cases/datasets-for-arpa-e-perform-program/)
 produced by [Adam Birchfield et al.](https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8423655) at Texas A&M with a