brief building instructions

README.md

@@ -14,4 +14,16 @@ Show below, there are three sets of APIs for different models.
 - Always choose the API at the *highest* possible level.
 - Refer to [the default modules](examples/pytorch/util.py) to see how to register message and node update functions as well as readout functions; note how you can control sharing of parameters by adding a counter.
 
+## How to build (the `cpp` branch)
 
+At the root directory of the repo:
+
+```sh
+$ mkdir build
+$ cd build
+$ cmake ..
+$ make
+$ export DGL_LIBRARY_PATH=$PWD/build
+```
+
+The `DGL_LIBRARY_PATH` environment variable should point to the library `libdgl.so` built by CMake.