Update README.md

Add JOSS DOI

README.md

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 [![DOI](https://zenodo.org/badge/513600791.svg)](https://zenodo.org/badge/latestdoi/513600791)
 
+[![DOI](https://joss.theoj.org/papers/10.21105/joss.06531/status.svg)](https://doi.org/10.21105/joss.06531)
+
 This code provides Finite Element solvers for electrochemical transport.
 Both continuous Galerkin (CG) and discontinuous Galerkin (DG) schemes are provided. The DG scheme for electroneutral Nernst-Planck is described in [Roy et al., 2023](https://doi.org/10.1016/j.jcp.2022.111859). The CG scheme uses SUPG for the advection-migration term.
 
 The following transport mechanisms are available: diffusion, advection, electromigration. EchemFEM supports both non-porous and porous cases. The ionic potential can either be described using an electroneutrality constraint or a Poisson equation. In the porous case, the electronic potential can be described by a Poisson equation.
 Some finite size effects are also implemented. For example, the generalized modified Poisson-Nernst-Planck model (GMPNP) is used in the examples of [FireCat](https://github.com/LLNL/firecat), coupling in with a microkinetics model.
 
+
 LLNL-CODE-837342
 
 ## Documentation
@@ -22,3 +25,4 @@ To install EchemFEM, simply run the following in the parent echemfem folder:
 ```
 pip install -e .
 ```
+The documentation has more details about installation and running the code.