docs(asm): open the assembly language reference

Assisted-by: GLM 5.3 Flash

README.md

@@ -186,8 +186,9 @@ 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 work has begun: [docs/GOOBJ.md](docs/GOOBJ.md)
-specifies the object file format completely. The assembler language
-reference follows.
+specifies the object file format completely, and
+[docs/asm/README.md](docs/asm/README.md) opens the language reference
+with its common core. The per-architecture pages follow.
 
 ## Direction
 
@@ -315,6 +316,7 @@ recipe.
   them
 - [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md): components and data flow
 - [docs/GOOBJ.md](docs/GOOBJ.md): the GOOBJ object file format specification
+- [docs/asm/](docs/asm/README.md): the Plan 9 assembly language reference
 - [docs/DEVELOPMENT.md](docs/DEVELOPMENT.md): development setup and recipes
 - [CHANGELOG.md](CHANGELOG.md): release history