Añade requisitos iniciales a readme

# Dockerd exporter

Prometheus exporter for Docker daemon metrics. Inspired by https://github.com/stefanprodan/swarmprom

## Requirements

You must enable Docker experimental metrics-addr in all Swarm nodes.

Edit `/etc/docker/daemon.json` to add these properties:

```json

{

	...

	"experimental": true,

	"metrics-addr": "0.0.0.0:9323"

}

```

Don't forget to restart Docker daemon with `systemctl restart docker` (do it for every Swarm node).