Update CONTRIBUTING.md to forewarn about the CLA requirement.
Update CONTRIBUTING.md to forewarn about the CLA requirement.

Per the suggestion from @schmod in https://github.com/apollographql/apollo-server/pull/2871#issuecomment-503157836, this adds a note that contributors will be expected to sign a Contributor License Agreement (CLA) when contributing to the Apollo Server project.
apollostack/apollo-server