docs: add usage

Author: rehiy

README.md

@@ -2,7 +2,16 @@
 
 This package implements the [libdns](https://github.com/libdns/libdns) interfaces for the [TencentCloud DNSPod API](https://www.tencentcloud.com/zh/document/api/1157/49025)
 
-## Applying for Security Credentials
+## Code example
+
+```shell
+provider := &tencentcloud.Provider{
+    SecretId:  "YOUR_Secret_ID",
+    SecretKey: "YOUR_Secret_Key",
+}
+```
+
+## Security Credentials
 
 To authenticate you need to supply a [TencentCloud API Key](https://console.tencentcloud.com/cam/capi).