XGBoost feature column support design doc (#2240)
XGBoost feature column support design doc (#2240)

* add sqlflow xgboost feature column support design doc

* follow comments
sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow