Release note of v0.3.0

[typhoonzero]

Please edit this issue to add release notes below:

• Support TO EVALUATE clause to evaluate a model.
• SQLFlow model zoo, support publicly share model definitions and models.

[typhoonzero]

• Support mathematical programming using SQL

[sneaxiy]

• Support feature column in XGBoost model, including training, evaluating, prediction and explaining.
• Support incremental training for both TensorFlow and XGBoost models.

[weiguoz]

• Add logs to record runtime status

[lhw362950217]

• Command-line Tool support release/remove model/repo
• Support SHOW TRAIN statement go get original SQL
• Create the SQLFlow Playground as a quick-start environment

[Yancey0623]

• Improvement
  • Improve the user experience on workflow mode, including improving workflow log structure, return selected rows, and diagnostic message to the GUI system.
  • Improve some diagnostic messages on the workflow model.
  • Supports passing all the selected columns into the prediction result table.
  • Decompose the all-in-one Docker image into separated Docker images.