Adding readme to LSTM code

Author: JAppleyard

posts/rnn/README.md

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+## Optimized LSTM forward pass in CUDA
+
+This code implements an LSTM forward pass in CUDA using cuBLAS. It uses cuRand to initialize the inputs and parameters.
+
+The PERFOPTS define allows the user to select which optimizations are enabled. It is a bitmask taking values from 0 to 31 with each bit defining whether a particular optimization is enabled.