tour: add Italian translation.

Change-Id: I387f441ace958b770630da2ac81dbe17a91fe6fc
Reviewed-on: https://go-review.googlesource.com/7042
Reviewed-by: Andrew Gerrand <adg@golang.org>

content/welcome.article

@@ -52,6 +52,7 @@ The tour is available in other languages:
 - [[http://go-tour-de1.appspot.com/][German — Deutsch]]
 - [[http://go-tour-es.appspot.com/][Spanish — Español]]
 - [[http://go-tour-fr.appspot.com/][French — Français]]
+- [[http://go-tour-ita.appspot.com/][Italian — Italiano]]
 - [[http://go-tour-he.appspot.com/][Hebrew — עִבְרִית]]
 - [[http://go-tour-jp.appspot.com/][Japanese — 日本語]]
 - [[http://go-tour-kr.appspot.com/][Korean — 한국어]]