update zenodo badge to 0.3.1

Author: wholmgren

README.md

@@ -5,7 +5,7 @@ pvlib-python
 [![Build status](https://ci.appveyor.com/api/projects/status/gr2eyhc84tvtkopk?svg=true)](https://ci.appveyor.com/project/wholmgren/pvlib-python-fv2to)
 [![Coverage Status](https://img.shields.io/coveralls/pvlib/pvlib-python.svg)](https://coveralls.io/r/pvlib/pvlib-python)
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+[![DOI](https://zenodo.org/badge/doi/10.5281/zenodo.50141.svg)](http://dx.doi.org/10.5281/zenodo.50141)
 
 
 pvlib-python is a community supported tool that provides a set of documented functions for simulating the performance of photovoltaic energy systems. The toolbox was originally developed in MATLAB at Sandia National Laboratories and it implements many of the models and methods developed at the Labs. More information on Sandia Labs PV performance modeling programs can be found at https://pvpmc.sandia.gov/. We collaborate with the PVLIB-MATLAB project, but operate independently of it.