added doc link

README.md

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+**[Documentation](http://rusty1s.github.io/pytorch_scatter)**
+
 This package consists of a small extension library of highly optimised sparse update (scatter) operations for the use in [PyTorch](http://pytorch.org/), which are missing in the main package.
 Scatter-operations can be roughly described as reduce-operations based on a given "group-index" tensor.
 The package consists of the following operations: