doc: improve specificity in CODEOWNERS
Update CODEOWNERS entry for README.md to only apply to the top level README.md. For example, if test/common/README.md is updated, that should not be assigned to TSC automatically.
Ref: https://github.com/nodejs/node/pull/20717#issuecomment-388909095
@richardlau
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documentation is changed or added -
commit message follows commit guidelines