[release-branch.go1.12] doc: document Go 1.11.6

Change-Id: I99832fa4f2c3ec28e2dad46cf7607f3766948031
Reviewed-on: https://go-review.googlesource.com/c/go/+/167698Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit d3bb45d9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/167701

doc/devel/release.html

@@ -80,6 +80,14 @@ the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.11.5">Go
 1.11.5 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.11.6 (released 2019/03/14) includes fixes to cgo, the compiler, linker,
+runtime, go command, and the <code>crypto/x509</code>, <code>encoding/json</code>,
+<code>net</code>, and <code>net/url</code> packages. See the
+<a href="https://github.com/golang/go/issues?q=milestone%3AGo1.11.6">Go
+1.11.6 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.10">go1.10 (released 2018/02/16)</h2>
 
 <p>