Added HINTS.md file for nth prime exercise. Issue exercism#209

exercises/nth-prime/HINTS.md

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+## Hints
+
+For this exercise the following C# feature comes in handy: 
+Enumerables are evaluated lazily.    They allow you to work with an infinite sequence of values.
+See [this article](https://blogs.msdn.microsoft.com/pedram/2007/06/02/lazy-evaluation-in-c/).
+
+Note: to help speedup calculation, you should not check numbers which you know beforehand will never be prime. For more information, see the [Sieve of Eratosthenes](https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes).