Add Vietnamese translation. (#212)

Author: freakboy3742

docs/config.yml

@@ -68,6 +68,9 @@ extra:
     - name: Türkçe
       link: /tr/
       lang: tr
+    - name: Tiếng Việt
+      link: /vi/
+      lang: vi
     - name: 中文(简体)
       link: /zh_CN/
       lang: zh-Hant