[GraphBolt] add example of heterograph node classification (#6339)

examples/graphbolt/rgcn/README.md

+# Node classification on heterogeneous graph with RGCN
+
+## Run on `ogbn-mag` dataset
+
+### Command
+```
+python3 hetero_rgcn.py
+```
+
+### Statistics of train/validation/test
+```
+Final performance: 
+All runs:
+Highest Train: 49.29 ± 0.85
+Highest Valid: 34.69 ± 0.49
+  Final Train: 48.14 ± 1.09
+   Final Test: 33.65 ± 0.63
+```
\ No newline at end of file