add warning for a bug in README (#2665)

examples/pytorch/graphsage/experimental/README.md

@@ -70,6 +70,10 @@ After runing this command, user can find a folder called ``graphsage`` on each m
 DGL provides a script to launch the training job in the cluster. `part_config` and `ip_config`
 specify relative paths to the path of the workspace.
 
+The command below launches one training process on each machine and each training process has 4 sampling processes.
+**Note**: There is a known bug in Python 3.8. The training process hangs when running multiple sampling processes for each training process.
+Please set the number of sampling processes to 0 if you are using Python 3.8.
+
 ```bash
 python3 ~/dgl/tools/launch.py \
 --workspace ~/graphsage/ \

examples/pytorch/rgcn/experimental/README.md

@@ -80,6 +80,10 @@ Users need to copy the training script to the workspace directory on remote mach
 DGL provides a script to launch the training job in the cluster. `part_config` and `ip_config`
 specify relative paths to the path of the workspace.
 
+The command below launches one training process on each machine and each training process has 4 sampling processes.
+**Note**: There is a known bug in Python 3.8. The training process hangs when running multiple sampling processes for each training process.
+Please set the number of sampling processes to 0 if you are using Python 3.8.
+
 ```bash
 python3 ~/dgl/tools/launch.py \
 --workspace ~/rgcn/ \