text: Title: Flask SQLAlchemy Integration - Documentation Suggestion
  Body: Firstly, thank you for the great extension!!
  
  I've ran into an error that I'm sure others will have ran into, it may be worth updating the docs with a warning about it.
  
  Our structure was as follows:
  
  - Each model has it's own module
  - Each model module also contains a Schema and Manager for example UserModel, UserSchema, UserManager all defined within /models/user.py
  
  Some background - with SQLAlchemy, with separate models, you need to import them all at runtime, before the DB is initialised to avoid circular dependancies within relationships.
  
  When the `UserSchema(ma.ModelSchema)` is hit during import `from app.models import *` (in bootstrap) this initialises the models and attempts to execute the relationships. At this stage, we may not have a relationship requirement (which SQLAlchemy avoids using string based relationships) however as the `ma.ModelSchema` initialises the models it creates errors such as this:
  
  > sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class User->users, expression ‘Team’ failed to locate a name (“name ‘Team’ is not defined”). If this is a class name, consider adding this relationship() to the <class ‘app.models.user.User’> class after both dependent classes have been defined.
  
  and, on subsequent loads:
  
  > sqlalchemy.exc.InvalidRequestError: Table ‘users_teams’ is already defined for this MetaData instance.  Specify ‘extend_existing=True’ to redefine options and columns on an existing Table object.
  
  The solution to this is to simply build the UserSchemas in a different import namespace, we've now got:
  ```
  /schemas/user_schema.py
  /models/user.py
  ```
  
  And no more circular issues - hopefully this helps someone else, went around in circles (pun intended) for a few hours before I realised it was the ModelSchema causing it.
  
  Could the docs be updated to make a point of explaining that the ModelSchema initialises the model, and therefore it's a good idea for them to be in separate import destinations?