doc: instructions for generating coverage reports

Add instructions for generating code coverage reports to BUILDING.md

PR-URL: https://github.com/nodejs/node/pull/15190


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>