perf: mention wiki page on dashboard

Otherwise no one will ever find it.

Change-Id: Ibcaf2df7d7b4a190dc7fa17f1bed92502830b6af
Reviewed-on: https://go-review.googlesource.com/c/build/+/563403
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

perf/app/dashboard/index.html

@@ -95,6 +95,10 @@ license that can be found in the LICENSE file.
 		Another way of reaching this page is to click the unit name that acts
 		as each graph's title, i.e. "sec/op".
 	</p>
+	<p>
+		Further documentation is available on the
+		<a href=https://go.dev/wiki/PerformanceMonitoring>wiki</a>.
+	</p>
 </div>
 
 <grid id="dashboard">