docs(README): add DartNode branding image to README

Author: cunoe

README.md

@@ -58,3 +58,5 @@ This extension solves the pain points of content creators when publishing across
     <img src="https://github.com/user-attachments/assets/1044058f-036e-4942-af07-e4bd48a1e1d6" width="350" alt="MultiPost Tencent Channel Wechat" />
   - QQ 扫一扫
     <img src="https://github.com/user-attachments/assets/fc9edf3b-1ed5-4aa7-99f9-b39dcab3f449" width="350" alt="MultiPost Tencent Channel QQ" />
+
+[![Powered by DartNode](https://dartnode.com/branding/DN-Open-Source-sm.png)](https://dartnode.com "Powered by DartNode - Free VPS for Open Source")