text: Title: Simple instructions for a self referential table
  Body: ### First Check
  
  - [X] I added a very descriptive title to this issue.
  - [X] I used the GitHub search to find a similar issue and didn't find it.
  - [X] I searched the SQLModel documentation, with the integrated search.
  - [X] I already searched in Google "How to X in SQLModel" and didn't find any information.
  - [X] I already read and followed all the tutorial in the docs and didn't find an answer.
  - [X] I already checked if it is not related to SQLModel but to [Pydantic](https://github.com/samuelcolvin/pydantic).
  - [X] I already checked if it is not related to SQLModel but to [SQLAlchemy](https://github.com/sqlalchemy/sqlalchemy).
  
  ### Commit to Help
  
  - [X] I commit to help with one of those options 👆
  
  ### Example Code
  
  ```python
  class Node(SQLModel, table=True):
      id: Optional[int] = Field(default=None, primary_key=True)
      text: str
      parent_id: Optional[int] = Field(foreign_key="node.id")
      # parent: Optional["Node"] not sure what to put here
      # children: List[Node] not sure what to put here either :)
  ```
  
  
  ### Description
  
  I am trying to create a simple self referential model - using the SQL model equivalent of the adjacency list pattern described here: https://docs.sqlalchemy.org/en/14/orm/self_referential.html
  
  I am only a litte familiar with SQL alchemy but was unable to translate their example into one that would work with SQLmodel.
  
  In your docs you said: "Based on SQLAlchemy [...] SQLModel is designed to satisfy the most common use cases and to be as simple and convenient as possible for those cases, providing the best developer experience". I was assuming that a self referential model would be a fairly common use case but totally appreciate that I could be wrong on this :)
  
  I see that there is an `sa_relationship` param that you can pass 'complicated stuff' too but I was not sure whether I should be using that (or how I would do so if I was meant to) - sorry just a bit too new to this.
  
  Crossing my fingers that it is straight forward to complete the commented lines in my example.
  
  ### Operating System
  
  Linux
  
  ### Operating System Details
  
  _No response_
  
  ### SQLModel Version
  
  0.0.4
  
  ### Python Version
  
  3.9.7
  
  ### Additional Context
  
  _No response_