Document the path to becoming a maintainer

[aaronshurley]

Describe the problem/challenge you have Currently, the path to becoming a maintainer isn't fully captured in our GOVERNANCE page. I'd like to better understand what the steps and details to becoming a maintainer consist of.

Describe the solution you'd like A detailed description of the roles, responsibilities, requirements, and processes of being a maintainer.

Anything else you would like to add: Community members, if y'all are interested in collaborating on this effort then please let us know in the comments!

Epic: https://github.com/vmware-tanzu/carvel/issues/161

[aaronshurley]

We are starting a Working Group that will meet until we publish a new contributor model. Please let us know if you'd like to be involved!