doc: add bartosz sosnowski to colaborators

Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: João Reis <reis@janeasystems.com>
PR-URL: https://github.com/nodejs/node/pull/7567