Add README.

Author: ilopX

patterns/visitor/conceptual/README.md

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+# Visitor pattern
+Visitor is a behavioral design pattern that lets you separate algorithms from the objects on which 
+they operate.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/visitor).
+
+### About example.
+
+## Client code:
+### Before:
+```dart
+void main() {
+  final list = createElements();
+  list.forEach(operation1);
+}
+
+Iterable<Object> createElements() {
+  return [
+    One(),
+    Two(),
+    Three(),
+  ];
+}
+
+void operation1(Object obj) {
+  if (obj is One) {
+    print('operation1: one (param1 = ${obj.param1})');
+  } else if (obj is Two) {
+    print('operation1: two (param2 = ${obj.param2})');
+  } else if (obj is Three) {
+    print('operation1: two (param3 = ${obj.param3})');
+  }
+}
+```
+
+### After:
+```dart
+void main() {
+  final list = createElements();
+
+  for (final e in elements) {
+    e.accept(visitor);
+  }
+}
+
+Iterable<Object> createElements() {
+  return [
+    One(),
+    Two(),
+    Three(),
+  ];
+}
+
+class ConcreteVisitor1 implements Visitor {
+  @override
+  void visitOne(One one) {
+    print('operation1: one (param1 = ${one.param1})');
+  }
+
+  @override
+  void visitTwo(Two two) {
+    print('operation1: two (param2 = ${two.param2})');
+  }
+
+  @override
+  void visitThree(Three three) {
+    print('operation1: three (param3 = ${three.param3})');
+  }
+}
+```
+#### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/174583542-5f57463c-148b-4113-acd3-2814ec017ecc.png)
+
+### Output:
+```
+operation1: one (param1 = 1)
+operation1: two (param2 = 2)
+operation1: three (param3 = 3)
+operation2: one (param1 = 1)
+operation2: two (param2 = 2)
+operation2: three (param3 = 3)
+```