add english version

en/code/src/apps/ch.2.5/attach_methods_to_struct/main.go

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+// Example code from Chapter 2.5
+// Attach method to struct.
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type Rectangle struct {
+	width, height float64
+}
+
+type Circle struct {
+	radius float64
+}
+
+func (r Rectangle) area() float64 {
+	return r.width * r.height
+}
+
+func (c Circle) area() float64 {
+	return c.radius * c.radius * math.Pi
+}
+
+func main() {
+	r1 := Rectangle{12, 2}
+	r2 := Rectangle{9, 4}
+	c1 := Circle{10}
+	c2 := Circle{25}
+
+	fmt.Println("Area of r1 is: ", r1.area())
+	fmt.Println("Area of r2 is: ", r2.area())
+	fmt.Println("Area of c1 is: ", c1.area())
+	fmt.Println("Area of c2 is: ", c2.area())
+}