Update README.md

Author: rodluger

README.md

@@ -7,6 +7,7 @@
   <a href="http://dx.doi.org/10.5281/zenodo.56577"><img src="https://zenodo.org/badge/doi/10.5281/zenodo.56577.svg" alt="10.5281/zenodo.56577"></a>
   <a href="https://raw.githubusercontent.com/rodluger/everest/master/LICENSE?token=AI5FKwMJER1BTPmm_2e_4VhVh3x3dfXHks5XfBCiwA%3D%3D"><img src="https://img.shields.io/badge/license-MIT-blue.svg"/></a>
   <a href="http://staff.washington.edu/rodluger/everest_docs"><img src="https://img.shields.io/badge/read-the_docs-red.svg?style=flat"/></a>
+  <a href="https://archive.stsci.edu/prepds/everest/"><img src="https://img.shields.io/badge/MAST-lightcurves-brightgreen.svg?style=flat"/></a>
 </p>
 
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