CONTRIBUTING: Make it more explicit you need to sign the CLA

[xrmx]

Description

Make it more explicit you need to sign the CLA and add link to the OpenTelemetry Contributor Guide.

Refs https://github.com/open-telemetry/opentelemetry-python-contrib/issues/2520

Once this goes I'll do the same for contrib in order to close the issue above.

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [ ] Test A

Does This PR Require a Contrib Repo Change?

• [ ] Yes. - Link to PR:
• [x] No.

Checklist:

• [ ] Followed the style guidelines of this project
• [ ] Changelogs have been updated
• [ ] Unit tests have been added
• [x] Documentation has been updated