perf: add brief dashboard overview

For golang/go#48803.

Change-Id: I15393e80fa9c2a961e7feacc3acd2b72de312b82
Reviewed-on: https://go-review.googlesource.com/c/build/+/413426
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

perf/app/dashboard/index.html

@@ -40,8 +40,22 @@ <h1>
 	</ul>
 </nav>
 
-<script>
-</script>
+<div class="Dashboard-documentation">
+	<p>
+		Each graph displays benchmark results relative to its baseline
+		commit, which is the latest stable release (e.g., 1.18.3) at
+		the time of testing. The 95% confidence interval is displayed
+		in light gray. On hover, the graph displays the benchmarked
+		commit at that point (click to view full commit).
+	</p>
+	<p>
+		Note that some commits are not tested, so there could be
+		multiple commits (not shown) between two points on the graph.
+		See the <code>linux-amd64-perf</code> column on the
+		<a href="https://build.golang.org/?repo=golang.org%2fx%2fbenchmarks">x/benchmarks build dashboard</a>
+		for tested ("ok") / untested (empty) commits.
+	</p>
+</div>
 
 <div id="dashboard">
 	<h2 class="Dashboard-title" id="loading">Loading...</h2>

perf/app/dashboard/static/style.css

@@ -93,6 +93,9 @@ nav a {
 .Dashboard-controls {
   padding: 0.5rem;
 }
+.Dashboard-documentation {
+  padding-left: 0.5rem;
+}
 
 .Dashboard-search {
   display: inline-block;