text: Title: Import error in example from README.md
  Body: When trying to create a type similar to the example in README.md, an error occurs:
  
  ![imagen](https://user-images.githubusercontent.com/58057324/120525607-ebe70600-c39d-11eb-8282-2a89eafff1e4.png)
  
  This is the example in README.md:
  
  ![imagen](https://user-images.githubusercontent.com/58057324/120525650-fb664f00-c39d-11eb-8903-741cf2bc7a81.png)
  
  The error can be solved by changing `import strawberry` to `import strawberry.django`, so I think README.md should be changed:
  
  ![imagen](https://user-images.githubusercontent.com/58057324/120525755-19cc4a80-c39e-11eb-827c-df31054f593d.png)
  