Document exceptaion with nyc configuration files

[tikolakin]

Spent hours to understand why coverage report is misbehaving and paths to files are not resolved correctly in HTML report - until read the code https://github.com/cypress-io/code-coverage/blob/d0130db664b1e3490ac686281ba974d28efd1315/task.js#L22 I had .nycrc file that wasn't taken into account

[CLAassistant]

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