[release-branch.go1.12] doc: add 1.12 to the project history

Go 1.12 is released, but it's currently not listed in the
https://golang.org/project page.

Change-Id: Ib5820f74245e4c986014c64eb40fa2911473e64b
Reviewed-on: https://go-review.googlesource.com/c/163837
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 467456b0afcbe8b74c49b356f0e5d41d1c6efc9f)
Reviewed-on: https://go-review.googlesource.com/c/163727
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/doc/contrib.html b/doc/contrib.html
index b4b19a6..fc853a9 100644
--- a/doc/contrib.html
+++ b/doc/contrib.html
@@ -34,6 +34,7 @@
 <p>A <a href="/doc/devel/release.html">summary</a> of the changes between Go releases. Notes for the major releases:</p>
 
 <ul>
+	<li><a href="/doc/go1.12">Go 1.12</a> <small>(February 2019)</small></li>
 	<li><a href="/doc/go1.11">Go 1.11</a> <small>(August 2018)</small></li>
 	<li><a href="/doc/go1.10">Go 1.10</a> <small>(February 2018)</small></li>
 	<li><a href="/doc/go1.9">Go 1.9</a> <small>(August 2017)</small></li>