Document complete list of current maintainers, including names, contact information, domain of responsibility, and affiliation.

[aburdenthehand]

One of the items on the CNCF Graduation checklist. @davidvossel @vladikr @rmohr @fabiand @stu-gott @vasiliy-ul @rthallisey I think github handle is fine for 'contact information' but we are missing 'domain of responsibility' for you all. Can you please note it here and we can update the maintainer file with a single PR.