Add v2 development warning to README

Add an important notice to the README indicating that the main branch
contains v2 of the SDK which is currently in development (pre-alpha).
The warning recommends v1.x for production use and links to the v1.x
branch for documentation and code.

This matches the warning added to the TypeScript SDK README.

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Author: maxisbey

README.md

@@ -13,6 +13,13 @@
 
 </div>
 
+> [!IMPORTANT]
+> **This is the `main` branch which contains v2 of the SDK (currently in development, pre-alpha).**
+>
+> We anticipate a stable v2 release in Q1 2026. Until then, **v1.x remains the recommended version** for production use. v1.x will continue to receive bug fixes and security updates for at least 6 months after v2 ships to give people time to upgrade.
+>
+> For v1 documentation and code, see the [`v1.x` branch](https://github.com/modelcontextprotocol/python-sdk/tree/v1.x).
+
 <!-- omit in toc -->
 ## Table of Contents