[docs] add Hummingbird to the list of external projects (#4096)

* add Hummingbird to the list of external projects

* Update README.md

README.md

@@ -70,6 +70,8 @@ JPMML (Java PMML converter): https://github.com/jpmml/jpmml-lightgbm
 
 Treelite (model compiler for efficient deployment): https://github.com/dmlc/treelite
 
+Hummingbird (model compiler into tensor computations): https://github.com/microsoft/hummingbird
+
 cuML Forest Inference Library (GPU-accelerated inference): https://github.com/rapidsai/cuml
 
 daal4py (Intel CPU-accelerated inference): https://github.com/IntelPython/daal4py