DOC: add v2.1.0 DOI

Author: tacaswell

doc/_templates/citing.html

@@ -29,6 +29,7 @@ <h1>Citing Matplotlib</h1>
 
 <h2>DOIs</h2>
 <dl>
+  <dt>v2.1.0</dt><dd><a href="https://doi.org/10.5281/zenodo.1004650"><img src="https://zenodo.org/badge/DOI/10.5281/zenodo.1004650.svg" alt="DOI"></a></dd>
   <dt>v2.0.2</dt><dd><a href="https://doi.org/10.5281/zenodo.573577"><img src="https://zenodo.org/badge/doi/10.5281/zenodo.573577.svg" alt="10.5281/zenodo.573577.svg"></a></dd>
   <dt>v2.0.1</dt><dd><a href="https://doi.org/10.5281/zenodo.570311"><img src="https://zenodo.org/badge/doi/10.5281/zenodo.570311.svg" alt="10.5281/zenodo.570311.svg"></a></dd>
   <dt>v2.0.0</dt><dd><a href="https://doi.org/10.5281/zenodo.248351"><img src="https://zenodo.org/badge/doi/10.5281/zenodo.248351.svg" alt="10.5281/zenodo.248351.svg"></a></dd>