content: add Russian translation of the tour.

Change-Id: I75d4745b2121fc848cf00eb4b3df22971ef4fd6f
Reviewed-on: https://go-review.googlesource.com/40790
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/content/welcome.article b/content/welcome.article
index a121688..c587172 100644
--- a/content/welcome.article
+++ b/content/welcome.article
@@ -63,6 +63,7 @@
 - [[https://go-tour-ua.appspot.com/][Ukrainian — Українська]]
 - [[https://go-tour-uz.appspot.com/][Uzbek — Ўзбекча]]
 - [[https://go-tour-turkish.appspot.com/][Turkish - Türkçe]]
+- [[https://go-tour-ru-ru.appspot.com/][Russian - Русский]]
 
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