Add new section on trait bounds.

exercises/04_traits/09_from/src/lib.rs

@@ -0,0 +1,10 @@
+// TODO: Implement the `From` trait for the `WrappingU32` type to make `example` compile.
+
+pub struct WrappingU32 {
+    value: u32,
+}
+
+fn example() {
+    let wrapping: WrappingU32 = 42.into();
+    let wrapping = WrappingU32::from(42);
+}