Add .env file usage to README

README.rst

@@ -231,6 +231,15 @@ To reset all existing secrets (secret environment variables) of a task definitio
 This will remove **all other** existing secret environment variables of **all containers** of the task definition, except for the new secret variable `NEW_SECRET` with the value coming from the AWS Parameter Store with the name "KEY_OF_SECRET_IN_PARAMETER_STORE" in the webserver container.
 
 
+Set environment via .env files
+==============================
+Instead of setting environment variables separately, you can pass a .env file per container to set the whole environment at once. You can either point to a local file or a file stored on S3, via::
+
+    $ ecs deploy my-cluster my-service --env-file my-app env/my-app.env
+
+    $ ecs deploy my-cluster my-service --s3-env-file my-app arn:aws:s3:::my-ecs-environment/my-app.env
+
+
 Set a docker label
 ===================
 To add a new or adjust an existing docker labels of a specific container, run the following command::