Update the contribution guidelines for Mbed TLS

This commit changes the contribution guidelines to refer the user to the
contribution guidelines in the Mbed TLS repo. It also removes the outdated references to the
CLA, and CLA not being required for trivial changes.

Signed-off-by: Simon Butcher <simon.butcher@arm.com>

Author: simonbutcher

features/mbedtls/README.md

@@ -85,9 +85,6 @@ For further documentation and help, you can visit the [Mbed TLS website](https:/
 
 ### Contributing to the project
 
-We are happy to accept bug reports and contributions from the community. There are some requirements to integrate contributions:
+We gratefully accept bug reports and contributions from the community. Contributions should be submitted to the [standalone Mbed TLS project](https://github.com/ARMmbed/mbedtls), not to the version of Mbed TLS embedded within Mbed OS.
 
-* Simple bug fixes to existing code do not contain copyright themselves, and we can integrate without issue. The same is true of trivial contributions.
-* For larger contributions, such as a new feature, the code can possibly fall under copyright law. We then need your consent to share in the ownership of the copyright. We have a form for this, which we will send to you if you submit a contribution or pull request that we deem this necessary for.
-
-Please submit contributions to the [standalone Mbed TLS project](https://github.com/ARMmbed/mbedtls), not to the version of Mbed TLS embedded within Mbed OS.
+Please see the [contributing guidelines](https://github.com/ARMmbed/mbedtls/blob/development/CONTRIBUTING.md) of the project for details on how to do this.