Add README.

Author: ilopX

patterns/builder/conceptual/README.md

@@ -0,0 +1,36 @@
+# Builder Pattern
+Builder is a creational design pattern that lets you construct complex objects step by step. The
+pattern allows you to produce different types and representations of an object using the same
+construction code.
+
+## Diagram:
+
+![image](https://user-images.githubusercontent.com/8049534/182850365-52969fc7-d743-430b-acc7-da400eae26aa.png)
+
+## Client code:
+
+```dart
+void main() {
+  final director = Director();
+
+  final product1 = director.construct(ConcreteBuilder1());
+  print(product1);
+
+  final product2 = director.construct(ConcreteBuilder2());
+  print(product2);
+}
+```
+
+### Output:
+
+```
+ConcreteBuilder1
+001: one
+002: two
+003: three
+
+ConcreteBuilder2
+1️⃣: first
+2️⃣: second
+3️⃣: third
+```