doc: mention net/http/httptrace package in release notes

Updates #15810

Change-Id: I689e18409a88c9e8941aa2e98f472c331efd455e
Reviewed-on: https://go-review.googlesource.com/23674Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.7.html

@@ -379,6 +379,13 @@ and the Go blog post
 “<a href="https://blog.golang.org/context">Go Concurrent Patterns: Context</a>.”
 </p>
 
+<h3 id="httptrace">HTTP Tracing</h3>
+
+<p>
+Go 1.7 introduces <a href="/pkg/net/http/httptrace/"><code>net/http/httptrace</code></a>,
+a package that provides mechanisms for tracing events within HTTP requests.
+</p>
+
 <h3 id="testing">Testing</h3>
 
 <p>