samples: rust: Add README for rust hello world

Create a Readme for this sample, explaining how to build and what it
does.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

samples/rust/hello_rust/README.rst

@@ -0,0 +1,35 @@
+.. _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`