doc: document the performance team

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

PR-URL: #12213
Refs: #12194 (comment)
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Joyee Cheung <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>

Author: gibfahn

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 |