Add VERSIONING.md documenting the versioning and breaking-change policy

VERSIONING.md

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+# Versioning Policy
+
+The MCP Ruby SDK follows [Semantic Versioning 2.0.0](https://semver.org/spec/v2.0.0.html).
+This document describes what each version component means for users of the `mcp` gem,
+what counts as a breaking change, and how breaking changes are communicated.
+
+## Version Scheme
+
+Given a version number `MAJOR.MINOR.PATCH`:
+
+- **MAJOR** is incremented for incompatible changes to the public API.
+- **MINOR** is incremented for backwards-compatible new functionality.
+- **PATCH** is incremented for backwards-compatible bug fixes.
+
+### Current 0.x Phase
+
+While the SDK is at version 0.x, the public API is not yet considered stable.
+Following SemVer conventions for initial development, minor releases (0.x -> 0.y) may contain breaking changes.
+All breaking changes are recorded in [CHANGELOG.md](CHANGELOG.md).
+
+### After 1.0.0
+
+Once 1.0.0 is released, breaking changes will only ship in major releases, with the exceptions described below.
+
+## What Counts as a Breaking Change
+
+A breaking change is any change that requires users to modify their code when upgrading, including:
+
+- Removing or renaming a public class, module, method, or constant
+- Changing a public method's signature in a way that rejects previously valid arguments
+- Changing documented behavior that users can reasonably depend on
+
+New functionality that is purely additive (new classes, new methods, new optional keyword arguments)
+is not a breaking change.
+
+## Exceptions: Spec Compliance, Security, and Clear Defects
+
+Minor releases avoid incompatible changes as much as possible. However, the SDK's primary contract is
+conformance to the [MCP specification](https://modelcontextprotocol.io/specification/).
+Behavior that deviates from the specification is treated as a bug, even when users may have come to rely on it.
+Therefore, incompatible changes may be shipped in a minor release when they are required to:
+
+- Fix incorrect conformance to the MCP specification
+- Address a security vulnerability
+- Fix a clear defect, such as behavior that contradicts the documentation, a crash,
+  or data corruption, where compatibility would mean preserving the defect
+
+When such a change ships, the release notes and CHANGELOG entry explicitly call out the incompatibility
+and describe how to migrate.
+
+## How Breaking Changes Are Communicated
+
+- [CHANGELOG.md](CHANGELOG.md) records every release in [Keep a Changelog](https://keepachangelog.com/en/1.1.0/) format;
+  breaking changes appear under the "Changed" or "Removed" headings
+- [GitHub Releases](https://github.com/modelcontextprotocol/ruby-sdk/releases) mirror the changelog for each release
+- Where practical, deprecation warnings are added at least one minor release before removal