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/163837Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contrib.html

@@ -34,6 +34,7 @@ We encourage all Go users to subscribe to
 <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>