docs not so good

hey guy, i just try flask-pw yesterday and after hours working, i give it up

the main reason why i give up is that docs is incomplete, for example:
- i don't know what migrate means? i just do it in `create` and `migrate` and then get an no such table error.
- what flask-pw would do when i change my model...

maybe something only for my careless, however i think docs should be more rich, and [readthedocs](https://readthedocs.org/) maybe a good place for it.

Review the existing documentation for the `create` and `migrate` commands, then check how model changes are currently described. Document what migration means, why the reported no-such-table error can occur, and what happens after changing a model; consider whether Read the Docs is the appropriate destination.

Written by the indexing model from the issue text.