Create github CODEOWNERS for core maintainer HIP management

[gjenkins8]

• make @helm/helm-org-maintainers default owner
• allow @helm/helm-org-maintainers and @helm/helm-core-maintainers (either may approve) for hips/ dir
• finally, ensure @helm/helm-org-maintainers remains CODEOWNERS owner

(for the CODEOWNERS to be fully valid, @helm/helm-core-maintainers needs to be added for write access; currently it does not)

[scottrigby]

Great work George.

Though I agree most HIPs are for Helm core, HIPs can and sometimes are proposals for other areas of the Helm project. hip-0001 makes clear who can approve a HIP:

The final authority for proposal approval is the project maintainers responsible for the project the proposal is aimed at

I do think this is an improvement, though I would just ask that Helm core maintainers bear in mind when a proposal is meant for another area of the Helm project.

Can you update this repo's CONTRIBUTING.md in this PR with the proposed changes including clarifying this note above so the process is clear?

To-do after merging:

• Once this PR merges, we will need to give @helm/helm-core-maintainers write access to this repo for this PR to have any effect.

[scottrigby]

Fun follow-up idea: hips could be in folders (or have a file naming convention, like hip-9999-AREA) to automatically map this to certain areas of the project, according to maintainer groups/subprojects. We could revisit after either #394 or #156 are merged.