README: Let people know a new version is coming (#98)

Add a note to the top that lets our community know that a new version of the extension is under development.

Author: akshayka

README.md

@@ -1,5 +1,8 @@
 # vscode marimo
 
+> [!Important]
+> **We are actively rewriting this extension to provide a more native and robust experience**, similar to what you may be used to when using Jupyter in VS Code. In the meantime, if you have issues with this extension, we recommend you run marimo from the terminal. See our [docs](https://docs.marimo.io/getting_started/) to get started.
+
 <a href="https://marketplace.visualstudio.com/items?itemName=marimo-team.vscode-marimo" target="__blank">
   <img src="https://img.shields.io/visual-studio-marketplace/v/marimo-team.vscode-marimo.svg?color=eee&amp;label=VS%20Code%20Marketplace&logo=visual-studio-code" alt="Visual Studio Marketplace Version" />
 </a>