Update README.md

README.md

@@ -12,6 +12,25 @@ Additional resources:
 - [SW4](https://geodynamics.org/resources/sw4/about)
 - [Forum](https://community.geodynamics.org/c/sw4/32)
 
+Cite Code As
+------------
+Petersson, N. Anders, Sjogreen, Bjorn, Tang, Houjun, & Pankajakshan, Ramesh. (2023, September 6). geodynamics/sw4: SW4, version 3.0. doi:10.5281/zenodo.8322590, url: https://doi.org/10.5281/zenodo.8322590
+
+Primary References
+------------------
+Zhang, L.;  Wang, S.; and Petersson, N.A. (2021), Elastic Wave Propagation in Curvilinear Coordinates with Mesh Refinement Interfaces by a Fourth Order Finite Difference Method, SIAM J. Sci. Comp.  43(2) pp. A1472-A1496. doi:10.1137/20M1339702
+
+Petersson, N.A.; Sjögreen, B. (2015), Wave propagation in anisotropic elastic materials and curvilinear coordinates using a summation-by-parts finite difference method, Journal of Computational Physics, 299, 820-841, doi: 10.1016/j.jcp.2015.07.023, url: http://linkinghub.elsevier.com/retrieve/pii/S0021999115004684
+
+Petersson, N.A.; Sjögreen, B. (2012), Stable and efficient modeling of anelastic attenuation in seismic wave propagation, Communications in Computational Physics, 12 (01), 193-225. doi:10.4208/cicp.201010.090611a
+
+Sjögreen, B.; Petersson, N.A. (2012), A Fourth Order Accurate Finite Difference Scheme for the Elastic Wave Equation in Second Order Formulation, Journal of Scientific Computing, 52 (1), 17-48, doi: 10.1007/s10915-011-9531-1, url: http://link.springer.com/10.1007/s10915-011-9531-1
+
+
+
+User's Guide
+------------
+Petersson, N.A.; Sjögreen, B.; Tang, H. (2023), User's Guide to SW4, version 3.0, LLNL-SM-741439
 
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