docs: better readme

README.md

@@ -3,7 +3,20 @@
 Uses DigitalOcean's API to generate a `file_sd_config`'s compatible JSON
 file, so you can service discovery DigitalOcean droplets on Prometheus.
 
-Example usage:
+## Usage
+
+You can run it as docker container:
+
+```sh
+docker run \
+  -v "$PWD":/tmp \
+  -e DO_TOKEN="digitalocean-read-only-token" \
+  --restart=always \
+  totvslabs/prometheus-digitalocean-sd --output.file=/tmp/do_sd.json
+```
+
+And then change your `prometheus.yml` config file to use `file_sd_configs`
+to read the `do_sd.json` file:
 
 ```yaml
 scrape_configs:
@@ -24,3 +37,26 @@ scrape_configs:
       - source_labels: [__meta_do_name]
         target_label: instance_name
 ```
+
+## File format
+
+The JSON file will have the following format and labels:
+
+```json
+[
+	{
+		"targets": [
+			"1.2.3.4:9100"
+		],
+		"labels": {
+			"__meta_do_az": "sfo1",
+			"__meta_do_id": "1234",
+			"__meta_do_name": "droplet-name",
+			"__meta_do_public_ip": "1.2.3.4",
+			"__meta_do_size": "4gb",
+			"__meta_do_status": "active"
+		}
+	},
+	// all other droplets
+]
+```