chore: Update README.

README.md

@@ -1,6 +1,6 @@
 # laurivan.grafana
 
-This role installs Grafana via Docker.
+This role installs Grafana, InfluxDB and Prometheus via Docker.
 
 ## Requirements
 
@@ -19,6 +19,7 @@ grafana_volume_base: "/mnt/data/grafana"
 grafana_setup_path: "{{ grafana_volume_base }}/config"
 grafana_volume_grafana: "{{ grafana_volume_base }}/grafana"
 grafana_volume_influxdb: "{{ grafana_volume_base }}/influxdb"
+grafana_volume_prometheus: "{{ grafana_volume_base }}/prometheus"
 ```
 
 You can however define specific volumes for the individual items:
@@ -26,6 +27,7 @@ You can however define specific volumes for the individual items:
 - `grafana_setup_path` - where the Docker compose files are placed
 - `grafana_volume_grafana` - volume for the grafana docker container
 - `grafana_volume_influxdb` - volume for InfluxDB
+- `grafana_volume_prometheus` - volume for Prometheus.io
 
 ### InfluxDB
 
@@ -40,6 +42,25 @@ influxdb:
   admin_token: change_me
 ```
 
+### Prometheus
+
+We define Prometheus variables:
+
+```yml
+prometheus_port: "9090"
+
+grafana_prometheus_project: my-project
+grafana_prometheus_job_name: prometheus
+grafana_prometheus_scrape_interval: '120s'
+grafana_prometheus_targets:
+  - "10.0.0.35:9100"
+  - "10.0.0.35:8080"
+```
+
+The targets are locations (ip address + port) where Prometheus polls for new data. This can be [cadvisor](https://github.com/google/cadvisor) or [node exporter](https://github.com/prometheus/node_exporter) for example.
+
+**Note**: Prometheus should be an internal service (i.e., empty `prometheus_port`). Hence, it doesn't have any security in place.
+
 ### Grafana
 
 We also define several variables for grafana