add example case

Author: wilfonba

examples/2D_riemann_test/README.md

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+# 2D Riemann Test (2D)
+
+Reference: Chamarthi, A., & Hoffmann, N., & Nishikawa, H., & Frankel S. (2023). Implicit gradients based conservative numerical scheme for compressible flows. arXiv:2110.05461 
+
+## Density Initial Condition
+
+![Density](alpha_rho1_initial.png)
+
+## Density Final Condition
+
+![Density Norms](alpha_rho1_final.png)