message: Update the README's clone command to clone submodules as well.
diff: @@ -29,7 +29,7 @@ This project is nascent and still under heavy development, but first steps:
  1. Clone the repository:
  ```shell
  - git clone https://github.com/tock/libtock-rs
  + git clone --recursive https://github.com/tock/libtock-rs
  cd libtock-rs
  ```
  