add new concat

Development/Go/strings.md

@@ -7,3 +7,23 @@ template := "My city is {city} and my country is {country}"
 r := strings.NewReplacer("{city}", "Sai Gon", "{country}", "Viet Nam")
 fmt.Println(r.Replace(template))
 ```
+
+Use `Builder` to **efficently** concat string:
+
+```go
+func oldConcat(strs ...string) string {
+	var result string
+	for _, str := range strs {
+		result += str
+	}
+	return result
+}
+
+func newConcat(strs ...string) string {
+	var sb strings.Builder
+	for _, str := range strs {
+		sb.WriteString(str)
+	}
+	return sb.String()
+}
+```