Document the env option to dj_database_url.config

[edmorley]

dj_database_url.config takes an env option that allows for customising the env var used for the database URL away from the default of DATABASE_URL:

dj-database-url/dj_database_url/__init__.py

Lines 65 to 66 in 7906f3b

	def config(
	env: str = DEFAULT_ENV,

However, this option isn't documented in the README:
https://github.com/jazzband/dj-database-url/blob/7906f3bf800105e6ab33bfbf44e6863fd178295a/README.rst#usage

Customising the env var is needed in the case of multiple DBs, where everything but the primary has to use something other than DATABASE_URL.

[lino]

I believe this issue has been resolved by your own contribution? Thank you.

[edmorley]

Hi! I'm not sure I follow?

This issue is about the env argument to dj_database_url.config not being documented in the README of this repo.

Checking the README now (using cmd+F -> search for "env") there is still no documentation of the env arg.