Add ROADMAP.md outlining the path toward SEP-1730 Tier 1

ROADMAP.md

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+# Roadmap
+
+This roadmap outlines the MCP Ruby SDK's path toward SEP-1730 Tier 1.
+It is a living document and will evolve as the SDK matures.
+
+## Current Status
+
+The SDK implements the 2025-06-18 and 2025-11-25 spec revisions, and passes all server and client conformance scenarios.
+
+## API Stability
+
+The 1.0.0 release will mark the public API as stable: breaking changes then ship only in major releases,
+apart from the narrow exceptions documented in [VERSIONING.md](VERSIONING.md),
+which also defines the Semantic Versioning scheme and breaking-change policy.
+
+## Deprecated Features
+
+The 2026-07-28 spec revision deprecates Roots, Sampling, and Logging (SEP-2577).
+These features remain fully supported throughout 1.x, and deprecation warnings will be added in a future minor release.
+Under Semantic Versioning their removal requires a major release, but no removal is scheduled yet:
+whether 2.0.0 removes them depends on how future MCP spec revisions treat these features
+and on adoption of their replacements.
+
+## Conformance
+
+The SDK maintains a 100% conformance pass rate as new scenarios are added to the conformance suite.
+Legacy SSE transport (2024-11-05) is intentionally out of scope; the SDK provides modern Streamable HTTP only.
+
+## Documentation
+
+Reference documentation covers all core features.
+
+## Tracking New Spec Revisions
+
+The SDK aims to support each new MCP specification revision, with the implementation timeline agreed per
+release based on feature complexity.
+Additive features from the 2026-07-28 revision (such as `server/discover`, multi round-trip requests,
+and the tasks extension) ship as opt-in functionality during 1.x. Breaking parts of that revision,
+such as the SEP-2575 stateless lifecycle rewrite, are reserved for 2.0.