doc: add example for running with v8-inspector

Add example to show what running Node.js with `--inspect`
should look like.

Some IDEs do not show the link when running with `--inspect`.
This example hints to what the full output looks like.

PR-URL: https://github.com/nodejs/node/pull/8845


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>