deps: add docs to upgrade openssl

This document is intended to describe the procedure to upgrade openssl
from 1.0.1m to 1.0.2a in io.js.

Fixes: https://github.com/iojs/io.js/issues/589
PR-URL: https://github.com/iojs/io.js/pull/1389


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>