doc: add domain postmortem

Do to various reasons, outlined in the committed document, domains were
only in core for 2 years before being deprecated. This outline explains
why they received criticism from the community and never gained traction
with module authors.

Also included is an example script that accompanies the postmortem
analysis.

PR-URL: https://github.com/nodejs/node/pull/6159


Reviewed-By: Robert Lindstädt <robert.lindstaedt@gmail.com>
Reviewed-By: Kelvin Knighton <keltheceo@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>