creational/builder: implement the builder pattern

creational/builder.md

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+# Builder Pattern
+
+Builder pattern separates the construction of a complex object from its
+representation so that the same construction process can create different
+representations.
+
+In Go, normally a configuration struct is used to achieve the same behavior,
+however passing a struct to the builder method fills the code with boilerplate
+`if cfg.Field != nil {...}` checks.
+
+## Implementation
+
+```go
+package car
+
+type Speed float64
+
+const (
+    MPH Speed = 1
+    KPH       = 1.60934
+)
+
+type Color string
+
+const (
+    BlueColor  Color = "blue"
+    GreenColor       = "green"
+    RedColor         = "red"
+)
+
+type Wheels string
+
+const (
+    SportsWheels Wheels = "sports"
+    SteelWheels         = "steel"
+)
+
+type Builder interface {
+    Color(Color) Builder
+    Wheels(Wheels) Builder
+    TopSpeed(Speed) Builder
+    Build() Interface
+}
+
+type Interface interface {
+    Drive() error
+    Stop() error
+}
+```
+
+## Usage
+
+```go
+assembly := car.NewBuilder().Paint(car.RedColor)
+
+familyCar := assembly.Wheels(car.SportsWheels).TopSpeed(50 * car.MPH).Build()
+familyCar.Drive()
+
+sportsCar := assembly.Wheels(car.SteelWheels).TopSpeed(150 * car.MPH).Build()
+sportsCar.Drive()
+```