perf_hooks: mark as experimental

PR-URL: nodejs/node#14997
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

doc/api/perf_hooks.md

@@ -3,6 +3,8 @@
 added: REPLACEME
 -->
 
+> Stability: 1 - Experimental
+
 The Performance Timing API provides an implementation of the
 [W3C Performance Timeline][] specification. The purpose of the API
 is to support collection of high resolution performance metrics.