Add a multi-target example config (#890)

Add an example Prometheus scrape config, similar to the
blackbox_exporter's example config.

Fixes: https://github.com/prometheus-community/postgres_exporter/issues/888

Signed-off-by: SuperQ <superq@gmail.com>

README.md

@@ -30,6 +30,26 @@ To use the multi-target functionality, send an http request to the endpoint `/pr
 
 To avoid putting sensitive information like username and password in the URL, preconfigured auth modules are supported via the [auth_modules](#auth_modules) section of the config file. auth_modules for DSNs can be used with the `/probe` endpoint by specifying the `?auth_module=foo` http parameter.
 
+Example Prometheus config:
+```yaml
+scrape_configs:
+  - job_name: 'postgres'
+    static_configs:
+      - targets:
+        - server1:5432
+        - server2:5432
+    metrics_path: /probe
+    params:
+      auth_module: [foo]
+    relabel_configs:
+      - source_labels: [__address__]
+        target_label: __param_target
+      - source_labels: [__param_target]
+        target_label: instance
+      - target_label: __address__
+        replacement: 127.0.0.1:9116  # The postgres exporter's real hostname:port.
+```
+
 ## Configuration File
 
 The configuration file controls the behavior of the exporter. It can be set using the `--config.file` command line flag and defaults to `postgres_exporter.yml`.