Add DOI badge for v2.2.1

Change-Id: Iee4acc67f47111af831926945227a198b6bd662d
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@@ -1,5 +1,7 @@
 # totalngrams
 
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.7471511.svg)](https://doi.org/10.5281/zenodo.7471511)
+
 Package for effectively processing token lists from very large corpora in tab separated value format, 
 by making full use of multicore-processors.