samples: rust: Add README for rust hello world

.. _hello_rust_world:

Hello Rust World

################

Overview

********

A simple :ref:`Rust <language_rust>` sample that can be used with many

supported boards and prints a Hello World message to the console.

Building and Running

********************

Once the proper rust toolchain has been installed.  This configuration

can be built and executed on QEMU as follows:

.. zephyr-app-commands::

   :zephyr-app: samples/rust/hello_rust

   :host-os: unix

   :board: qemu_riscv32

   :goals: run

   :compact:

To build for another board, change "qemu_riscv32" above to that

board's name.

Sample Output

=============

.. code-block: console

   Hello world from Rust on qemu_riscv32

Exit QEMU by pressing :kbd:`CTRL+C`