domain: add soft deprecation notice

Per the TC meeting on 2014-12-10, domains will be "soft deprecated"
until a suitable replacement API is available; at which time they
will be fully deprecated. Full deprecation will include references
to replacement API and the application of util.deprecate to the domain
api.

PR-URL: https://github.com/iojs/io.js/pull/141
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>