Move README.md to top level (pytorch#22)

* Create README.md

* Delete README.MD

torchrec/README.MD → README.md

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-# Torchrec
+# TorchRec (🚨 Warning: Unstable Prototype 🚨)
 
 TorchRec is a PyTorch domain library built to provide common sparsity & parallelism primitives needed for large-scale recommender systems (RecSys). It allows authors to train models with large embedding tables sharded across many GPUs.