Extended compilation instructions C++ API (#157)

* complemented compilation instructions C++ API
Signed-off-by: Felix Pultar <felix.pultar@phys.chem.ethz.ch>

* complemented compilation instructions C++ API
Signed-off-by: Felix Pultar <felix.pultar@phys.chem.ethz.ch>

* Update README.md
Signed-off-by: Felix Pultar <felix.pultar@phys.chem.ethz.ch>
Co-authored-by: Matthias Fey <matthias.fey@tu-dortmund.de>

README.md

@@ -284,6 +284,7 @@ pytest
 `torch-cluster` also offers a C++ API that contains C++ equivalent of python models.
 
 ```
+export Torch_DIR=`python -c 'import torch;print(torch.utils.cmake_prefix_path)'`
 mkdir build
 cd build
 # Add -DWITH_CUDA=on support for the CUDA if needed