Added readme for the PyTorch directory.

pytorch/README.md

+# Notes
+
+The source code sample was taken from the Horovod examples repository 
+[here](https://github.com/horovod/horovod/tree/master/examples/pytorch) 
+(last checked: April 27, 2021). The sample has been slightly modified. Our 
+changes are limited to,
+
+*  The data loading mechanism.
+*  Removal of `filelock` to eliminate dependence on a package that is not 
+   available on the supercomputers.
+*  A few additional comments pertaining to our custom data loading mechanism.
+
+**Note:** All newly added statements follow a comment beginning with `[HPCNS]`. 
+All statements that demonstrate the use of Horovod follow a comment beginning 
+with `[Horovod]` (as added by Horovod developers).
+
+The following sample is included:
+
+1.  `mnist.py`: A simple training program for an MNIST classifier that 
+    uses Horovod for data distribution.