Drop requirement for a CLA

The Mbed TLS project no longer requires a CLA. Contributions from now on
must be made under both Apache-2.0 AND GPL-2.0-or-later licenses, to enable
LTS (Long Term Support) branches of the software to continue to be provided
under either Apache-2.0 OR GPL-2.0-or-later. Contributors must accept the
terms of the Developer Certificate of Origin (DCO) by adding a Signed-off-by:
line to each commit message.

The software on the development branch continues to be provided under
Apache-2.0.

Update README.md, CONTRIBUTING.md and pull_request_template.md to explain
the new licensing model. Add a copy of the DCO to the project.

Expand the full Apache-2.0 license text in the LICENSE file and remove the
redundant apache-2.0.txt.

Signed-off-by: Dan Handley <dan.handley@arm.com>
diff --git a/README.md b/README.md
index 16ba188..83c0fe1 100644
--- a/README.md
+++ b/README.md
@@ -191,21 +191,12 @@
 -   [What external dependencies does Mbed TLS rely on?](https://tls.mbed.org/kb/development/what-external-dependencies-does-mbedtls-rely-on)
 -   [How do I configure Mbed TLS](https://tls.mbed.org/kb/compiling-and-building/how-do-i-configure-mbedtls)
 
+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. 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.
+
 Contributing
 ------------
 
-We gratefully accept bug reports and contributions from the community. There are some requirements we need to fulfill in order to be able to integrate contributions:
-
--   All contributions, whether large or small require a Contributor's License Agreement (CLA) to be accepted. This is because source code can possibly fall under copyright law and we need your consent to share in the ownership of the copyright.
--   We would ask that contributions conform to [our coding standards](https://tls.mbed.org/kb/development/mbedtls-coding-standards), and that contributions should be fully tested before submission.
--   As with any open source project, contributions will be reviewed by the project team and community and may need some modifications to be accepted.
-
-To accept the Contributor’s Licence Agreement (CLA), individual contributors can do this by creating an Mbed account and [accepting the online agreement here with a click through](https://os.mbed.com/contributor_agreement/). Alternatively, for contributions from corporations, or those that do not wish to create an Mbed account, a slightly different agreement can be found [here](https://www.mbed.com/en/about-mbed/contributor-license-agreements/). This agreement should be signed and returned to Arm as described in the instructions given.
-
-### Making a Contribution
-
-1.  [Check for open issues](https://github.com/ARMmbed/mbedtls/issues) or [start a discussion](https://forums.mbed.com/c/mbed-tls) around a feature idea or a bug.
-2.  Fork the [Mbed TLS repository on GitHub](https://github.com/ARMmbed/mbedtls) to start making your changes. As a general rule, you should use the "development" branch as a basis.
-3.  Write a test which shows that the bug was fixed or that the feature works as expected.
-4.  Send a pull request and bug us until it gets merged and published. Contributions may need some modifications, so work with us to get your change accepted. We will include your name in the ChangeLog :)
-
+We gratefully accept bug reports and contributions from the community. Please see the [contributing guidelines](CONTRIBUTING.md) for details on how to do this.