text: Title: Document the use of pytest with dynaconf
  Body: For testing in my project i want to add in my conftest.py something like that:
  
  ```
  import pytest
  import os
  
  @pytest.fixture(scope='session', autouse=True)
  def settings():
      os.environ['ENV_FOR_DYNACONF'] = 'testing'
  
  ```
  But this is not work ;-(. What can you advise me ?
  I dont want start my test like that : `ENV_FOR_DYNACONF=testing pytest` because somebody can miss that command prefix and mess up some dev data. 