[docs] subdirectories and description for control plane (#4506)

* subdirectories and description for control plane

* changed the description for the google/rpc subdirectory in the source code orgnization tree, made it more precise

docs/README.md

@@ -19,6 +19,7 @@ p4c
 │   ├── tc                    -- translates P4 code to Linux TC rules
 │   └── ubpf                  -- translates P4 programs to uBPF code
 ├── control-plane             -- control plane API
+|   └──google/rpc             -- Definitions of Google Protobuf APIs.
 ├── docs                      -- documentation
 │   └── doxygen               -- documentation generation support
 ├── extensions