Warn the user about NOT deleting containers (#12)

* Warn the user about NOT deleting containers

As the `supervisor` does not support creating the homeassistant container if accidentally deleted, let's warn the users before it's too late and perhaps give a bare minimum of possible ways to recover.

* Update README.md

README.md

@@ -57,3 +57,20 @@ curl -sL https://raw.githubusercontent.com/home-assistant/hassio-installer/maste
 - raspberrypi3
 - raspberrypi3-64
 - tinker
+
+## !!!WARNING!!! DO NOT DELETE CREATED CONTAINERS
+
+This installer will create the base `homeassistant` container for you, but if you delete if (`docker rm` or `docker prune`) the supervisor will not be able to re-create it for you. Hass.io is a Eco System and Container Orchastrator they don't support manual adjustments.
+
+If you wish to still safelly use `docker containers prune` you might want to add the `--filter` flag to your command.
+
+Example:
+```sh
+$ docker container prune --filter label!=homeassistant
+```
+or, for a greater peace of mind, you can add the following config to your `~/.docker/config.json`:
+```javascript
+{
+  "pruneFilters": ["label!=homeassistant", "label!=hassio_supervisor", "label!=addon*"]
+}
+```