Update README.md

Author: friedenhe

README.md

@@ -6,7 +6,7 @@ DAFoam: Discrete Adjoint with OpenFOAM
 DAFoam develops an efficient discrete adjoint method to perform high-fidelity multidisciplinary design optimization. DAFoam has the following features:
 
 - It uses a popular open-source package [OpenFOAM](https://www.openfoam.com) for multiphysics analysis.
-- It implements an efficient discrete adjoint approach with competitive speed, scalability, accuracy, and compatibility.
+- It implements a [Jacobian-free discrete adjoint](https://www.sciencedirect.com/science/article/abs/pii/S0376042119300120) approach with competitive speed, scalability, and accuracy.
 - It has a convenient Python interface to couple with [OpenMDAO](https://openmdao.org) for multidisciplinary design optimization.
 
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