Added configuration for russian translation.

_config.yml

@@ -12,7 +12,7 @@ description: Lessons about the Elixir programming language
 baseurl: /
 url: https://elixirschool.com
 permalink: pretty
-languages: ['en', 'jp', 'cn', 'es', 'pt', 'vi']
+languages: ['en', 'jp', 'cn', 'es', 'pt', 'vi', 'ru']
 default_lang: en
 exclude_from_localization: []
 sections:
@@ -24,6 +24,7 @@ sections:
  es: Básico
  pt: Básico
  vi: Cơ bản
+ ru: Основы
  - tag: advanced
  label:
  en: Advanced
@@ -32,6 +33,7 @@ sections:
  es: Avanzado
  pt: Avançado
  vi: Nâng cao
+ ru: Продвинутый
  - tag: specifics
  label:
  en: Specifics
@@ -40,6 +42,7 @@ sections:
  es: Específico
  pt: Específico
  vi: Cụ thể
+ ru: Специфика
 
 gems:
  - kramdown