Added documentation in the readme

README.md

@@ -63,6 +63,30 @@ Start provisioning of the cluster using the following command:
 ansible-playbook playbooks/site.yml -i inventory.yml
 ```
 
+### Using an external database
+
+If an external database is preferred, this can be achieved by passing the `--datastore-endpoint` as an extra server argument as well as setting the `use_external_database` flag to true.
+
+```bash
+k3s_cluster:
+  children:
+    server:
+      hosts:
+        192.16.35.11:
+        192.16.35.12:
+    agent:
+      hosts:
+        192.16.35.13:
+
+  vars:
+    use_external_database: true
+    extra_server_args: "--datastore-endpoint=postgres://username:password@hostname:port/database-name"
+```
+
+The `use_external_database` flag is required when more than one server is defined, as otherwise an embedded etcd cluster will be created instead.
+
+The format of the datastore-endpoint parameter is dependent upon the datastore backend, please visit the [K3s datastore endpoint format](https://docs.k3s.io/datastore#datastore-endpoint-format-and-functionality) for details on the format and supported datastores.
+
 ## Upgrading
 
 A playbook is provided to upgrade K3s on all nodes in the cluster. To use it, update `k3s_version` with the desired version in `inventory.yml` and run: