South Korean actor Jang Dong-yoon has announced that he will marry actress Kim Seung-yoon in October. The 34-year-old broke the news in a handwritten letter posted on Instagram today (Aug 25), saying that they plan to hold a quiet ceremony with their families. He wrote: "There were so many thoughts and worries before deciding to get married. By spending a lot of time together, we came to know each other's many different sides, and in turn, we became more understanding of and cherished each other. "Now, we have decided to support each other as our closest companion and live together happily and joyfully." He acknowledged that the news may…