Add Serial Link lesson to part 2

Author: quinnyo

src/SUMMARY.md

@@ -30,6 +30,7 @@
 - [Collision](part2/collision.md)
 - [Bricks](part2/bricks.md)
 - [Decimal Numbers](part2/bcd.md)
+- [Serial Link](part2/serial-link.md)
 - [Work in progress](part2/wip.md)
 
 # Part III — Our second game