docs: Add beta status notice to README (#2556)

Clarifies that ty is currently in beta and explains the 0.0.x version
numbering.
This addresses issue #2462 by providing clear communication about the
project's
beta status directly in the README.

Closes #2462

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

README.md

@@ -21,6 +21,8 @@ An extremely fast Python type checker and language server, written in Rust.
 ty is backed by [Astral](https://astral.sh), the creators of
 [uv](https://github.com/astral-sh/uv) and [Ruff](https://github.com/astral-sh/ruff).
 
+ty is currently in [beta](#version-policy).
+
 ## Highlights
 
 - 10x - 100x faster than mypy and Pyright
@@ -67,6 +69,12 @@ to anything in the `ruff` submodule (which includes all of the Rust source code)
 See the
 [contributing guide](./CONTRIBUTING.md) for more details.
 
+## Version policy
+
+ty uses `0.0.x` versioning. ty does not yet have a stable API; breaking changes, including changes
+to diagnostics, may occur between any two versions. See the [type system support](https://github.com/astral-sh/ty/issues/1889)
+tracking issue for a detailed overview of currently supported features.
+
 ## FAQ
 
 <!-- We intentionally use smaller headings for the FAQ items -->