doc: explains why Reviewed-By is added in PRs

Adds verbose reasons to the documentation on why the
Reviewed-By metadata on a pull request is important.
This was loosely mentioned as an issue in the referenced
issue below, and answered by @addaleax.

Ref: https://github.com/nodejs/node/issues/8893
PR-URL: https://github.com/nodejs/node/pull/9044


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>