docs(goobj): document the GOOBJ object file format

README.md

@@ -185,9 +185,9 @@ is written as that knowledge is produced during development. It is
 verified the way the code is verified: an encoding documented here is
 one that differential tests against `go tool asm` confirm
 byte-for-byte, and a format field documented here is one the linker
-demonstrably reads. The result will live in this repository, so that
-Plan 9 assembly finally carries a reference its own tooling is built
-against.
+demonstrably reads. The work has begun: [docs/GOOBJ.md](docs/GOOBJ.md)
+specifies the object file format completely. The assembler language
+reference follows.
 
 ## Direction
 
@@ -314,6 +314,7 @@ recipe.
   ~/.local/share/man (MANDIR overrides); `just uninstall-man` removes
   them
 - [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md): components and data flow
+- [docs/GOOBJ.md](docs/GOOBJ.md): the GOOBJ object file format specification
 - [docs/DEVELOPMENT.md](docs/DEVELOPMENT.md): development setup and recipes
 - [CHANGELOG.md](CHANGELOG.md): release history