Add a note about starting listmonk without a config file.

docs/docs/content/configuration.md

@@ -6,7 +6,9 @@ One or more TOML files can be read by passing `--config config.toml` multiple ti
 To generate a new sample configuration file, run `--listmonk --new-config`
 
 ### Environment variables
-Variables in config.toml can also be provided as environment variables prefixed by `LISTMONK_` with periods replaced by `__` (double underscore). Example:
+Variables in config.toml can also be provided as environment variables prefixed by `LISTMONK_` with periods replaced by `__` (double underscore). To start listmonk purely with environment variables without a configuration file, set the environment variables and pass the config flag as `--config=""`.
+
+Example:
 
 | **Environment variable**       | Example value  |
 | ------------------------------ | -------------- |