doc: document Go 1.12.6

Change-Id: I8ae00d2392c20c627d58cf7e79015e982b971802
Reviewed-on: https://go-review.googlesource.com/c/go/+/181551Reviewed-by: Filippo Valsorda <filippo@golang.org>

doc/devel/release.html

@@ -69,6 +69,14 @@ the go command, the runtime, and the <code>os</code> package. See the
 1.12.5 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.12.6 (released 2019/06/11) includes fixes to the compiler, the linker,
+the go command, and the <code>crypto/x509</code>, <code>net/http</code>, and
+<code>os</code> packages. See the
+<a href="https://github.com/golang/go/issues?q=milestone%3AGo1.12.6">Go
+1.12.6 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>