doc: document Go 1.9.3

Change-Id: Ic7a5d3118754b34ab0652fcef889259a03baebc3
Reviewed-on: https://go-review.googlesource.com/88536

Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/devel/release.html

@@ -49,6 +49,14 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.9.2">Go
 1.9.2 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.9.3 (released 2018/01/19) includes fixes to the compiler, runtime,
+and the <code>database/sql</code>, <code>math/big</code>, <code>net/http</code>,
+and <code>net/url</code> packages.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.9.3">Go
+1.9.3 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.8">go1.8 (released 2017/02/16)</h2>
 
 <p>