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>

COLLABORATOR_GUIDE.md

@@ -101,6 +101,8 @@ information regarding the change process:
 
 - A `Reviewed-By: Name <email>` line for yourself and any
   other Collaborators who have reviewed the change.
+  - Useful for @mentions / contact list if something goes wrong in the PR.
+  - Protects against the assumption that GitHub will be around forever.
 - A `PR-URL:` line that references the *full* GitHub URL of the original
   pull request being merged so it's easy to trace a commit back to the
   conversation that led up to that change.