add design doc: xgboost on sqlflow (#579)
add design doc: xgboost on sqlflow (#579)

* add desgin doc: xgboost on sqlflow

* add distributed part

* fix some comments

* reorganize some parts

sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow,sql-machine-learning/sqlflow