Update the README's clone command to clone submodules as well.

README.md

diff --git a/README.md b/README.md
index e4b0eee..52ea0b6 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@
 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
     ```