docs: Add motivation and example usage to README

README.md

@@ -1,5 +1,32 @@
 # terraform-grafana-prometheus-alerts
-Terraform module to convert Prometheus alert rules to Grafana alerts
+
+Terraform module to convert [Prometheus Alerting rules] to Grafana-managed alerts
+
+## Motivation / Why using this module
+
+There are plenty of apps (mostly out of CNCF's ecosystem) where the vendor or the community provides monitoring dashboards
+and alerts. Dashboards are normally provided as a JSON file which can be loaded into Grafana. Alerts are mostly provided
+as [Prometheus Alerting rules].
+
+There are users who already operate a Grafana instance or use a managed Grafana instance from a cloud provider (Grafana
+Cloud, Amazon Managed Grafana, Azure Managed Grafana, etc.). Why not using this Grafana instance for the
+alerting?
+
+The problem is that Grafana's unified alerting uses another format for the alert definition but the concept with labels,
+annotations (provide description and runbook URLs) is almost identical.
+This module allows you to reuse the [Prometheus Alerting rules] and configure them inside Grafana.
+
+## Example usage
+
+```hcl
+module "cert_manager_rules" {
+  source = "github.com/mkilchhofer/terraform-grafana-prometheus-alerts"
+
+  prometheus_alerts_file_path = file("/path/to/alerts/cert-manager.yaml")
+  folder_uid                  = grafana_folder.test.uid
+  datasource_uid              = grafana_data_source.victoria_metrics.uid
+}
+```
 
 ## Requirements
 
@@ -41,3 +68,5 @@ TODO
 | alertsfile\_map | n/a |
 | file\_as\_yaml | n/a |
 <!-- END_TF_DOCS -->
+
+[Prometheus Alerting rules]: https://prometheus.io/docs/prometheus/latest/configuration/alerting_rules/