Add DOI [no CI].

Author: jpivarski

README.md

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 Python implementation of Histogrammar
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+[![DOI](https://zenodo.org/badge/doi/10.5281/zenodo.61418.svg)](http://dx.doi.org/10.5281/zenodo.61418)
+
 See [histogrammar.org](http://histogrammar.org) for a complete introduction to Histogrammar.
 
 This is a pure Python implementation for Python versions 2.6, 2.7, and 3.4.