docs: add deprecation info

Author: seanwu1105

README.md

@@ -1,5 +1,7 @@
 # Visual Studio Code Extension Qt for Python
 
+> ⚠️ This project is deprecated. Use [the official Qt Extensions](https://marketplace.visualstudio.com/items?itemName=TheQtCompany.qt) instead.
+
 [![build](https://github.com/seanwu1105/vscode-qt-for-python/workflows/build/badge.svg)](https://github.com/seanwu1105/vscode-qt-for-python/actions?query=workflow:build)
 [![version](https://img.shields.io/visual-studio-marketplace/v/seanwu.vscode-qt-for-python.svg)](https://marketplace.visualstudio.com/items?itemName=seanwu.vscode-qt-for-python)
 [![rating](https://img.shields.io/visual-studio-marketplace/r/seanwu.vscode-qt-for-python.svg)](https://marketplace.visualstudio.com/items?itemName=seanwu.vscode-qt-for-python)