Add CLA requirement to CONTRIBUTING.md

[alex-bain]

Describe the bug

The CONTRIBUTING.md file does not contain information that signing the Contributor License Agreement is a requirement for contributing to NebulaGL.

Actual Result

After opening a PR, the CLAAssistant bot asks a developer to sign a CLA before a PR can be merged.

Expected Result

The CONTRIBUTING.md file contains information that signing a CLA is a requirement for contributing to NebulaGL.

[ibgreen]

@alex-bain Looks like this repo has just been locked down - all contributors (which are nowadays all external to Uber) have lost write access.

For now we are moving development to a fork https://github.com/visgl/nebula.gl. That should not have any CLA integration. Also invited you as contributor there.