doc: document the performance team

Can be cc'ed with `@nodejs/performance`.

PR-URL: https://github.com/nodejs/node/pull/12213
Refs: https://github.com/nodejs/node/pull/12194#issuecomment-291477258


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>