Updates README with DOI (#680)

The badge DOI will always resolve to the newest release that has been pushed / identified on zenodo. I have manually uploaded the 3.6.0 source from 11.13.2018.  As new releases are peeled off they should be identified and good to go.

README.md

@@ -1,5 +1,6 @@
 # ISIS3
 
 [![Join the chat at https://gitter.im/USGS-Astrogeology/isis3_cmake](https://badges.gitter.im/USGS-Astrogeology/isis3_cmake.svg)](https://gitter.im/USGS-Astrogeology/isis3_cmake?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.2525679.svg)](https://doi.org/10.5281/zenodo.2525679)
 
 Integrated Software for Imagers and Spectrometers ISIS3 public release site