add fibonacci

fibonacci/README.md

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+# Exercise XX - fibonacci
+
+Create a function that returns a specific member of the fibonacci sequence:
+
+> a series of numbers in which each number ( Fibonacci number ) is the sum of the two preceding numbers. The simplest is the series 1, 1, 2, 3, 5, 8, etc.
+
+```javascript
+fibonacci(4) // returns the 4th member of the series: 3  (1, 1, 2, 3)
+fibonacci(6) // returns 8
+```