Document how to add environment variables

[malparty]

Why

Once deployed, a project often needs to add environment variables.
The template already provides a file structure to do that, but it is not documented 🙃

Who Benefits?

Future developers won't need to ask veterans on slack 🪖

How?

We can add a section in the README.md file directly, after the starter-kit (#83 ):

## Add environment Variables

> TODO: Describe how to add env variables using the file `service.json.tftpl` in the ECS module.

[malparty]

@hoangmirs would you be interested in handling it? 😇 (tho it might worth to wait #83 to be reviewed so we avoid conflicts)