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: https://github.com/jazzband/dj-database-url/blob/7906f3bf800105e6ab33bfbf44e6863fd178295a/dj_database_url/init.py#L65-L66

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.