Wysihtml5

http://jhollingworth.github.com/bootstrap-wysihtml5/

Add gem 'bootstrap-wysihtml5-rails' to your Gemfile

Then update config file config/initializers/rails_admin.rb

RailsAdmin.config do |config|
  config.model Team do
    edit do
      # For RailsAdmin >= 0.5.0
      field :description, :wysihtml5
      # For RailsAdmin < 0.5.0
      # field :description do
      #   bootstrap_wysihtml5 true
      # end
    end
  end
end

# To configure the editor bar pass a hash of options:
# For RailsAdmin >= 0.5.0
field :description, :wysihtml5 do
  config_options :html => true
end   

More here