Add README.

Author: ilopX

patterns/singleton/conceptual/README.md

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+# Singleton Pattern
+Singleton is a creational design pattern that lets you ensure that a class has only one instance, 
+while providing a global access point to this instance.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/singleton).
+
+### Diagram:
+![Singleton Pattern Diagram](https://user-images.githubusercontent.com/8049534/182938119-78a21534-5751-4dea-afa3-8acaec46eed9.png)
+
+### Client code:
+```dart
+void main() {
+  // dart style
+  Singleton().doSome();
+  Singleton().doSome();
+
+  // standard style
+  Singleton.instance.doSome();
+}
+```
+
+### Output:
+```
+Create singleton once.
+doSome()
+doSome()
+doSome()
+```