add Singleton pattern

singleton/README.md

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+## Singleton
+
+Ensure a class only has one instance, and provide a global point of access to it.
+Pattern has creational purpose and deals with object relationships, which are more 
+dynamic. The Singleton is often used as a part another design patterns (see [Fa<46>ade] 
+and [Flyweight]). 
+
+### When to use
+
+* there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point
+* when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code
+
+[Fa<46>ade]: https://github.com/JakubVojvoda/design-patterns-cpp/tree/master/facade
+[Flyweight]: https://github.com/JakubVojvoda/design-patterns-cpp/tree/master/flyweight
+