Add authoritative public API catalog

README.md

@@ -14,6 +14,11 @@ complete public API shim.
 The source snapshot, compatibility rules, dependency research, and ordered
 implementation plan are in [RUSTREWRITE.md](RUSTREWRITE.md).
 
+The authoritative compiled .NET surface is checked in as
+[`api/public-api.json`](api/public-api.json); [`api/README.md`](api/README.md)
+documents deterministic regeneration and the external types that must be
+mapped to native Rust equivalents.
+
 ```sh
 cargo build --workspace
 cargo test --workspace --no-run