Need docs for model usage: general and model specific

[hungry1526]

Is your feature request related to a problem? Please describe. Currently there is no doc for models we used in the example. We should find a place to include generic and model specific instructions for new users and experienced ones.

Describe the solution you'd like

• Include example in Github MD files and SQLFlow.org
• Include usage in command line
• Discuss how do we maintain this kind of information, for open sourced models, and customized models.

Describe alternatives you've considered (This is optional) A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.

[typhoonzero]

We have https://github.com/sql-machine-learning/sqlflow/blob/develop/doc/model_parameter.md yet the supported model list document is still needed.