add Builder pattern

builder/README.md

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+## Builder
+
+Builder pattern has creational purpose and separates the construction of a complex object 
+from its representation so that the same construction process can create different 
+representations. It is object pattern, ie. relationships can be changed at run-time
+and are more dynamic. Often is used for building composite structures but constructing
+objects requires more domain knowledge of the client than using a Factory.
+
+### When to use
+
+* the algorithm for creating a object should be independent of the parts and how they're assembled
+* the construction process must allow different representations for the object that's constructed