Add custom help info

Signed-off-by: Xiang Dai <764524258@qq.com>

production/helm/README.md

@@ -18,10 +18,18 @@ $ helm repo update
 
 ## Deploy Loki and Promtail to your cluster
 
+### Deploy with default config
+
 ```bash
 $ helm upgrade --install loki loki/loki-stack
 ```
 
+### Deploy with custom config
+
+```bash
+$ helm upgrade --install loki loki/loki --set "key1=val1,key2=val2,..."
+```
+
 ## Deploy Loki only
 
 ```bash