created a README

Author: tangentstorm

README.md

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+# sudoku-solver
+
+This program treats a sudoku puzzle as a [set cover problem](https://en.wikipedia.org/wiki/Set_cover_problem)
+and then solves it using Knuth's [dancing links algorithm](https://en.wikipedia.org/wiki/Dancing_Links).
+
+See [this blog article](https://gieseanw.wordpress.com/2011/06/16/solving-sudoku-revisited/) for an
+in-depth explanation of the idea.