rune/skeleton: add protoc-c dependency in README
Fixes:
@@ -6,6 +6,9 @@ Note that this code base is inspired by [v28 SGX in-tree driver](https://patchwo
# Run skeleton with Docker
## Build liberpal-skeleton
+
+`skeleton` depends on `protobuf-c` compiler. Please refer to [this step](https://github.com/protobuf-c/protobuf-c#building) to install it on your platform.
+
```shell
cd "${path_to_inclavare_containers}/rune/libenclave/internal/runtime/pal/skeleton"
make
