sim: Update doc to explain submodules
Now that we depend on a submodule for mbed TLS code, update the docs to
explain how to fetch this.  Otherwise, the error is somewhat misleading,
just showing a missing sha256.c file.
@@ -16,6 +16,16 @@ simulator can be built with the stable release of Rust.
.. _installation: https://www.rust-lang.org/en-US/install.html
+Dependent code
+--------------
+
+The simulator depends on some external modules. These are stored as
+submodules within git. To fetch these dependencies the first time::
+
+ $ git submodule update --init
+
+will clone and check out these trees in the appropriate place.
+
Building
========
