add C++ description

Author: rusty1s

README.md

@@ -281,6 +281,19 @@ print(valueC)
 tensor([8.0, 6.0, 8.0])
 ```
 
+## C++ API
+
+`torch-sparse` also offers a C++ API that contains C++ equivalent of python models.
+
+```
+mkdir build
+cd build
+# Add -DWITH_CUDA=on support for the CUDA if needed
+cmake ..
+make
+make install
+```
+
 ## Running tests
 
 ```