Adding change of SECRET_KEY in README to be able to run project locally

[jaswdr]

When I tried to run the project locally, I have received this error:

...
    if settings.USE_I18N:
  File "/home/jaswdr/.local/share/virtualenvs/censo-querido-diario--OuRbTDe/lib/python3.8/site-packages/django/conf/__init__.py", line 83, in __getattr__
    self._setup(name)
  File "/home/jaswdr/.local/share/virtualenvs/censo-querido-diario--OuRbTDe/lib/python3.8/site-packages/django/conf/__init__.py", line 70, in _setup
    self._wrapped = Settings(settings_module)
  File "/home/jaswdr/.local/share/virtualenvs/censo-querido-diario--OuRbTDe/lib/python3.8/site-packages/django/conf/__init__.py", line 196, in __init__
    raise ImproperlyConfigured("The SECRET_KEY setting must not be empty.")
django.core.exceptions.ImproperlyConfigured: The SECRET_KEY setting must not be empty.

This happens because the SECRET_KEY has an empty string value in the censo/settings.py, I'm adding a section in the README mentioning this in this PR.