Don't warn on NOTE comments

[cassidyjames]

PR #75 introduced warnings for FIXME, TODO, and NOTE comments. The former two were what were requested in #48, as both FIXME and TODO imply a non-ideal or unfinished state that a developer might want to address before committing.

However, NOTE should not elicit a warning as it's perfectly fine to add clarifying comments. We don't want to discourage that with warnings.

[jeremypw]

It could be argued that a clarifying comment does not need to be marked "NOTE" (all comments are for the purpose of clarification of some kind). This should only be used for particularly important comments.