Update README regarding currently used JavaScript compilation levels
Update README regarding currently used JavaScript compilation levels

The README file stated that Debug and Release modes corresponded to
different modes of the Closure Compiler, while this is not true anymore.

googlechromelabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector