Add a note about I18n.available_locales into README
Add a note about I18n.available_locales into README

It closes #6

SpaceCowboyMax/rails_admin_globalize_field,SpaceCowboyMax/rails_admin_globalize_field,mitsuru/rails_admin_globalize_field,scarfacedeb/rails_admin_globalize_field,mitsuru/rails_admin_globalize_field,mitsuru/rails_admin_globalize_field,SpaceCowboyMax/rails_admin_globalize_field,glebtv/rails_admin_hstore_translate,scarfacedeb/rails_admin_globalize_field,rwd/rails_admin_globalize_field,rwd/rails_admin_globalize_field,rwd/rails_admin_globalize_field,scarfacedeb/rails_admin_globalize_field