doc: document the performance team

Can be cc'ed with @nodejs/performance.

I added some people: @mscdex @vsemozhetbyt @joyeecheung @TimothyGu @lpinca

If anyone else wants to join and be cc/ed on issues, feel free to request access (or remove yourself if you don't want to be on the team).

Link to team

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

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)