Add note on pytorch implementation to readme

Author: tscohen

README.md

@@ -1,4 +1,6 @@
 
+### Note: If you are looking for a PyTorch implementation, please have a look at the pull requests by Jorn Peters and Adam Bielski (https://github.com/tscohen/GrouPy/pulls).
+
 # GrouPy
 
 GrouPy is a python library that implements group equivariant convolutional neural networks [\[Cohen & Welling, 2016\]](#gcnn) in Chainer and TensorFlow, and supports other numerical computations involving transformation groups.