Update documentation

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/README.md b/README.md
index a3fcd2e..956d8ba 100644
--- a/README.md
+++ b/README.md
@@ -307,7 +307,7 @@
 License
 -------
 
-Unless specifically indicated otherwise in a file, Mbed TLS files are provided under the [Apache-2.0](https://spdx.org/licenses/Apache-2.0.html) license. See the [LICENSE](LICENSE) file for the full text of this license, and [the 'License and Copyright' section in the contributing guidelines](CONTRIBUTING.md#License-and-Copyright) for more information.
+Unless specifically indicated otherwise in a file, Mbed TLS files are provided under a dual [Apache-2.0](https://spdx.org/licenses/Apache-2.0.html) OR [GPL-2.0-or-later](https://spdx.org/licenses/GPL-2.0-or-later.html) license. See the [LICENSE](LICENSE) file for the full text of these licenses, and [the 'License and Copyright' section in the contributing guidelines](CONTRIBUTING.md#License-and-Copyright) for more information.
 
 ### Third-party code included in Mbed TLS