text: Title: Document the export_schema command
  Body: ## Feature Request Type
  
  - Documentation
  
  ## Description
  
  I was figuring out how to export my schema, since the new version of graphiql doesn't seem to allow you. It was a bit of a struggle. With graphene there was a management command to do it; and the penny dropped that this should have the same.
  
  Little did I know that there was already an `export_schema` command. Let's put it in the docs!