Putting back mysql2 and sqlite3 in database.yml docs. In case people use "--skip-gemfile"

railties/lib/rails/generators/rails/app/templates/config/databases/mysql.yml

 # MySQL.  Versions 4.1 and 5.0 are recommended.
+# 
+# Install the MYSQL driver
+#   gem install mysql2
 #
 # Ensure the MySQL gem is defined in your Gemfile
 #   gem 'mysql2'

railties/lib/rails/generators/rails/app/templates/config/databases/sqlite3.yml

 # SQLite version 3.x
+#   gem install sqlite3
+#
 #   Ensure the SQLite 3 gem is defined in your Gemfile
 #   gem 'sqlite3'
 development: