Add a v2 status banner to the README

States that v2 is in alpha with pre-releases published to PyPI as
2.0.0aN, that installers never select a pre-release without an explicit
opt-in, that packages depending on mcp should add a <2 upper bound
before the stable v2 release, and that v1.x is in maintenance mode and
remains recommended for production. This README is also the PyPI
project description for stable releases, so the banner uses absolute
links and plain blockquote markup.

Author: maxisbey

README.md

@@ -13,6 +13,12 @@
 
 </div>
 
+> **This documents v1.x, the stable release line of the MCP Python SDK. v2 is in alpha.**
+>
+> v2 pre-releases are published to PyPI as `2.0.0aN`. Installers never select a pre-release unless you opt in (for example `pip install mcp==2.0.0a1`), so v1.x users are unaffected. **If your package depends on `mcp`, add a `<2` upper bound to your version constraint (for example `mcp>=1.27,<2`) before the stable v2 release lands.** See the [v2 documentation](https://github.com/modelcontextprotocol/python-sdk/blob/main/README.v2.md) and the [migration guide](https://github.com/modelcontextprotocol/python-sdk/blob/main/docs/migration.md) for what's changing.
+>
+> v1.x remains recommended for production use. It is in maintenance mode and continues to receive critical bug fixes and security patches.
+
 <!-- omit in toc -->
 ## Table of Contents