Document BASE_DIR setting.

[Eric Baumgartner]

BASE_DIR appears to be an established setting. The default startapp template defines it.

​https://github.com/django/django/blob/master/django/conf/project_template/project_name/settings.py-tpl

And third-party apps like django_extensions rely on BASE_DIR being set.

However, the main settings documentation page does not mention BASE_DIR at all.

​https://docs.djangoproject.com/en/3.0/ref/settings/

This led to confusion in the following case:

• Working with a legacy project that started around django 0.9.6 and has never defined BASE_DIR, to no apparent ill effect.
• While using django_extensions, an exception is raised ('Settings' object has no attribute 'BASE_DIR').
• Think: "Ok, that's fine, just need to define it, it's probably a newer setting."
• Go to django docs to see the definition of BASE_DIR. Find nothing!
• Now confused. Is BASE_DIR unofficial? Why is django_extensions relying on a non-standard setting?

It would be nice if BASE_DIR could be added to the settings documentation to resolve this confusion.

[Mariusz Felisiak]

BASE_DIR is not a setting. It's a constant set and used only by a default settings.py template.