Add root readme for Atomic Blaster linking to original XNA tutorial

AtomicNET/AtomicBlaster/README.md

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+### Atomic Blaster
+
+Atomic Blaster is a port of ShapeBlaster for XNA by Michael Hoffman.
+
+Read the XNA tutorial at Tuts+: http://gamedevelopment.tutsplus.com/articles/make-a-neon-vector-shooter-for-ios-first-steps--gamedev-14316