docs: add README with project overview and ecosystem links - #13

Summary

Adds a root README as part of the org-wide discoverability pass. The repository previously had no README, About description, or topics.

• One-liner: automatic gRPC contract generation for Cratis Arc backends via protobuf-net — verified against the source (Generator scans [Command]/[Query]/[ObservableQuery] types, Server routes through Arc's command/query pipelines).
• Describes the Generator, Generator.Build (MSBuild), Server, and sample projects, linking the existing per-project READMEs.
• Labels the repository as an exploration, consistent with spec.md.
• Adds a short Cratis ecosystem section (Arc, Chronicle, Fundamentals) and license note.

No code changes.