Clarify env file passing

This is a bit unclear in current explanations but seems quite important if you're moving to docker from an existing install.

docker/multi-process/README.md

@@ -92,10 +92,12 @@ The `docker/multi-process` folder also has a `docker-compose.yml` that allows fo
 
 ## Environment Variables
 
-Other Huginn 12factored environment variables of note, as generated and put into the .env file as per Huginn documentation. All variables of the [.env.example](https://github.com/huginn/huginn/blob/master/.env.example) can be used to override the defaults which a read from the current `.env.example`.
+Other Huginn [12factored](https://12factor.net/) environment variables of note are generated and put into the .env file as per Huginn documentation. All variables of the [.env.example](https://github.com/huginn/huginn/blob/master/.env.example) can be used to override the defaults which a read from the current `.env.example`.
 
 For variables in the .env.example that are commented out, the default is to not include that variable in the generated .env file.
 
+In newer versions of Docker you are able to pass your own .env file in to the container with the `--env-file` parameter.
+
 ## Building on your own
 
 You don't need to do this on your own, because there is an [automated build](https://registry.hub.docker.com/u/huginn/huginn/) for this repository, but if you really want run this command in the Huginn root directory: