docs: describe Ruby interop adapter contract

[EfeDurmaz16]

Summary

• document a small process-oriented Ruby adapter contract for cross-language MPP conformance
• map the proposed client and server adapters to existing mpp-rb transport, middleware, and example primitives
• link the interop note from the README

Closes #12

Verification

• git diff --check
• ruby -e 'Dir["docs/**/*.md", "README.md"].each { |p| abort("missing #{p}") unless File.file?(p); s = File.read(p); abort("tab in #{p}") if s.include?("\\t") }; puts "markdown files readable"'