Document our MSRV policy (#11898)

## Summary

Copying astral-sh/ruff#16384 over to uv.

Author: charliermarsh

docs/reference/policies/versioning.md

@@ -18,3 +18,13 @@ Cache versions are considered internal to uv, and so may be changed in a minor o
 The `uv.lock` schema version is considered part of the public API, and so will only be incremented
 in a minor release as a breaking change. See
 [Lockfile versioning](../../concepts/resolution.md#lockfile-versioning) for more.
+
+## Minimum supported Rust version
+
+The minimum supported Rust version required to compile uv is listed in the `rust-version` key of the
+`[workspace.package]` section in `Cargo.toml`. It may change in any release (minor or patch). It
+will never be newer than N-2 Rust versions, where N is the latest stable version. For example, if
+the latest stable Rust version is 1.85, uv's minimum supported Rust version will be at most 1.83.
+
+This is only relevant to users who build uv from source. Installing uv from the Python package index
+usually installs a pre-built binary and does not require Rust compilation.