docs(extensions): add basic doc for extensions.

Author: phodal

exts/README.md

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+# AutoDev advanced Extension
+
+In this folder, you can find some advanced extensions for AutoDev.
+
+- Android. This extension allows you to build Android with GenAI ability.
+- Database. Build an SQL, Oracle or MySQL database with GenAI ability.
+- Vue. Build a Vue.js application with GenAI ability.
+
+## How to add new extensions
+
+follow [https://ide.unitmesh.cc/development/dev-new-language.html](https://ide.unitmesh.cc/development/dev-new-language.html)