Add example to show how to use external secrets (#744)

Signed-off-by: Pete Wall <pete.wall@grafana.com>

examples/external-secrets/README.md

@@ -0,0 +1,46 @@
+# External Secrets
+
+Using external secrets allows you to store the hostnames, usernames, passwords and other sensitive data in a pre-made
+Kubernetes Secret object. This example shows how to reference that secret, as well the keys within it:
+
+```yaml
+cluster:
+  name: external-secrets-test
+externalServices:
+  prometheus:
+    hostKey: prom_hostname
+    secret:
+      create: false
+      name: alloy-secret
+      namespace: monitoring
+    basicAuth:
+      usernameKey: prom_username
+      passwordKey: password
+
+  loki:
+    hostKey: loki_hostname
+    secret:
+      create: false
+      name: alloy-secret
+      namespace: monitoring
+    basicAuth:
+      usernameKey: loki_username
+      passwordKey: password
+```
+
+here is the secret object itself: 
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: alloy-secret
+  namespace: monitoring
+type: Opaque
+stringData:
+  prom_hostname: "https://prometheus.example.com"
+  prom_username: "12345"
+  loki_hostname: "https://loki.example.com"
+  loki_username: "12345"
+  password: "It's a secret to everyone"
+```