Add DOI of JOSS paper

Author: firefly-cpp

README.md

@@ -15,6 +15,8 @@
 ![GitHub commit activity](https://img.shields.io/github/commit-activity/w/firefly-cpp/niaarm.svg)
 [![Average time to resolve an issue](http://isitmaintained.com/badge/resolution/firefly-cpp/niaarm.svg)](http://isitmaintained.com/project/firefly-cpp/niaarm "Average time to resolve an issue")
 
+[![DOI](https://joss.theoj.org/papers/10.21105/joss.04448/status.svg)](https://doi.org/10.21105/joss.04448)
+
 
 NiaARM is a framework for Association Rule Mining based on nature-inspired algorithms for optimization. The framework is written fully in Python and runs on all platforms. NiaARM allows users to preprocess the data in a transaction database automatically, to search for association rules and provide a pretty output of the rules found. This framework also supports integral and real-valued types of attributes besides the categorical ones. Mining the association rules is defined as an optimization problem, and solved using the nature-inspired algorithms that come from the related framework called [NiaPy](https://github.com/NiaOrg/NiaPy).