[v1.x] Add a v2 status banner to the README by maxisbey · Pull Request #2835 · modelcontextprotocol/python-sdk

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.