Path to Code Owner

[tlattner]

The path to code owner is not documented.

We should revisit project wide policies/documentation on how code owners are nominated, how someone can set out on a path to become a code owner, and determine how to ensure that we are casting a wide net of candidates.

[kbeyls]

I believe this issue might be fixed as part of the move to "maintainers" rather than "code owners", and the documentation that @AaronBallman wrote at https://llvm.org/docs/DeveloperPolicy.html#maintainers