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>

doc/onboarding-extras.md

@@ -27,6 +27,7 @@
 | `test/*` | @nodejs/testing |
 | `tools/eslint`, `.eslintrc` | @not-an-aardvark, @silverwind, @trott |
 | async_hooks | @nodejs/diagnostics |
+| performance | @nodejs/performance |
 | upgrading V8 | @nodejs/v8, @nodejs/post-mortem |
 | upgrading npm | @fishrock123, @MylesBorins |
 | upgrading c-ares | @jbergstroem |