Update README.md

README.md

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 | [**GitHub**](https://github.com/bessagroup/CRATE)
 | [**PyPI**](https://pypi.org/project/cratepy/)
 
+[![DOI](https://joss.theoj.org/papers/10.21105/joss.05594/status.svg)](https://doi.org/10.21105/joss.05594)
+
 ### Summary
 **CRATE** (Clustering-based Nonlinear Analysis of Materials) is a Python project (package [cratepy](https://pypi.org/project/cratepy/)) developed in the context of computational mechanics to aid the design and development of new materials. Its main purpose is **performing multi-scale nonlinear analyses of heterogeneous materials** through a suitable coupling between first-order computational homogenization and clustering-based reduced-order modeling: given a representative volume element of the material microstructure and the corresponding material phase properties, CRATE computes the material's effective mechanical response when subject to a prescribed macro-scale loading path.