Update documentation

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

Author: daverodgman

CONTRIBUTING.md

@@ -84,11 +84,11 @@ Mbed TLS is well documented, but if you think documentation is needed, speak out
 License and Copyright
 ---------------------
 
-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.
+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. This means that users may choose which of these licenses they take the code under.
 
 Contributors must accept that their contributions are made under both the Apache-2.0 AND [GPL-2.0-or-later](https://spdx.org/licenses/GPL-2.0-or-later.html) licenses. This enables LTS (Long Term Support) branches of the software to be provided under either the Apache-2.0 or GPL-2.0-or-later licenses.
 
-All new files should include the [Apache-2.0](https://spdx.org/licenses/Apache-2.0.html) standard license header where possible.
+All new files should include the standard SPDX license identifier where possible, i.e. "SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later".
 
 The copyright on contributions is retained by the original authors of the code. Where possible for new files, this should be noted in a comment at the top of the file in the form: "Copyright The Mbed TLS Contributors".