Update doc for building on OSX (#5074)
Update doc for building on OSX (#5074)


Co-Authored-By: Jiaming Yuan <e16b25b7883ebdfb0cca6b409cf5afe1de1589fd@outlook.com>

dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost