Add logging guidelines

[liu-cong]

This is the first attempt to define logging guidelines in our current code base. This is not meant to be perfect/permanent, but something that can improve our logging quickly.

I am happy to follow up with a PR to update logging once this is approved.

Related issue: #108

cc @danehans @ahg-g

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[kfswain]

This looks reasonable. Can we give a crisp name to each log level?

That would let us make some enums that we can expect to be used as the log level params. It'll give us semantics within the code that makes readability, and following this style decision a tad easier.