added zenodo badge to readme

README.md

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 # Pyxem template matching workflows
 
+[![DOI](https://zenodo.org/badge/425115743.svg)](https://zenodo.org/badge/latestdoi/425115743)
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 This repository contains all the Jupyter notebook files and other files necessary to reproduce the results and figures in the paper *"Free, flexible and fast: orientation mapping using the multi-core and GPU-accelerated template matching capabilities in the python-based open source 4D-STEM analysis toolbox Pyxem"* which was recently submitted to the journal *Ultramicroscopy*; arXiv preprint details are pending.
 
 What these notebooks show: