I recently bought an ST-E3-RT and 600EX-RT and was thinking of buying another 600EX-RT when a friend told me about his Yongnuo flashes and what great bang for the buck they are. Are there any Yongnuo flashes that can be fired via radio signal by the ST-E3-RT? If not, what is my next best option? I'm new to flash photography, so please be gentle.