doc: added information on how to run the linter.

Added clarification about the linter execution.

PR-URL: https://github.com/nodejs/node/pull/7534


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>