Add more explicit co-maintainers

[ndmitchell]

Various personal and health stuff has meant I haven't been that on top of this repo. Given the backlog that accumulates in my absence, I think I need co-maintainers. Given existing contributions in the recent past I've openned this up to @shayne-fletcher and @zliu41. You both have permission to merge things. Whether you choose to use that permission, or become official co-maintainers, is entirely up to you.

[zliu41]

Hope you get better soon!