README suggestion: link to dotenv to explain `.env` file

[xpe]

I think a link in the README would help this section:

For convenience, you can use a .env file to set DATABASE_URL so that you don't have to pass it every time:

DATABASE_URL=mysql://localhost/my_database

The README can be improved: it shouldn't assume that people know what a .env file is.

(For example, many people use direnv. They might wonder if the above line was a typo for .envrc.)

I see that sqlx uses dotenv in the Cargo.toml file. So I would suggest the README link directly to dotenv. This allows people to easily find it and read more about it.