Contribution guidelines do not mention CLA requirement under runtime contributions

[fweimer-rh]

The older contribution guidelines do not mention the CLA requirement the first time contributing to the Mono runtime is mentioned here:

Contributing to the Mono runtime: If you are submitting changes to the runtime, you must make sure to release your change under the MIT license before it is integrated. You may do so by adding this line to your commit messages:

This change is released under the MIT license.

I didn't see the CLA reference further down when I prepared #21730. For such a trivial change, it would have been easier for everyone just to describe the required change verbally in an issue.