Merge branch 'k-devise' into 'master'

added note about Devise

Added note about the authentication gem used in GitLab

Because of question in https://groups.google.com/forum/#!topic/gitlabhq/G2rToah19-Q

See merge request !912

Gemfile

@@ -9,7 +9,7 @@ gem "default_value_for", "~> 3.0.0"
 gem "mysql2", group: :mysql
 gem "pg", group: :postgres
-# Auth
+# Authentication libraries
 gem "devise", '3.2.4'
 gem "devise-async", '0.9.0'
 gem 'omniauth', "~> 1.2.2"