perf_hooks: mark as experimental

PR-URL: #14997
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Timothy Gu <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Jeremiah Senkpiel <[email protected]>

Author: jasnell

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.