doc: document Go 1.12

Change-Id: I845375d2b3824211b80885228ba5b45503cba1a6
Reviewed-on: https://go-review.googlesource.com/c/163722Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/devel/release.html

@@ -23,6 +23,13 @@ in supported releases as needed by issuing minor revisions
 (for example, Go 1.6.1, Go 1.6.2, and so on).
 </p>
 
+<h2 id="go1.12">go1.12 (released 2019/02/25)</h2>
+
+<p>
+Go 1.12 is a major release of Go.
+Read the <a href="/doc/go1.12">Go 1.12 Release Notes</a> for more information.
+</p>
+
 <h2 id="go1.11">go1.11 (released 2018/08/24)</h2>
 
 <p>