Maintainer file specification - per TOC Offsite

During the TOC offsite, it was proposed and discussed to introduce more structure to the MAINTAINERS file in CNCF Project repositories.

Creating a consistent structure in representing the maintainers and their affiliation has the potential to enable better metrics for indicating distribution of activity by maintainers and categories of maintainers affiliation. The former allows the TOC to match declared maintainers with ongoing activity in the repository in order to more easily ascertain lottery factor, active maintainers, and freshness of maintainer listing. The latter allows the TOC to understand project investment by affiliation category into the projects' overall direction and health.

Categories of affiliation discussed include but are not limited to:
* End user (note this does not need to be a CNCF End User)
* Independent - individuals whose maintainership and participation is completely unrelated to their employer
* Academic
* Government
* Vendor
* Non-profit