sim: Create a small README.rst
Describe how to build and run the simulator.
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+MCUboot Simulator
+#################
+
+This is a small simulator designed to exercise the mcuboot upgrade
+code, specifically testing untimely reset scenarios to make sure the
+code is robust.
+
+Prerequisites
+=============
+
+The simulator is written in Rust_, and you will need to install it to
+build it. The installation_ page describes this process. The
+simulator can be built with the stable release of Rust.
+
+.. _Rust: https://www.rust-lang.org/
+
+.. _installation: https://www.rust-lang.org/en-US/install.html
+
+Building
+========
+
+Once Rust is installed, build cargo by::
+
+ $ cargo build --release
+
+this should download and compile the necessary dependencies, compile
+the relevant modules from mcuboot, and build the simulator. The
+resulting executable will be placed in ``./target/release/bootsim``
+and can be run directly::
+
+ $ ./target/release/bootsim run --device k64f
+
+Calling with ``--help`` will give a more thorough usage.
+
+Debugging
+=========
+
+If the simulator indicates a failure, you can turn on additional
+logging by setting ``RUST_LOG=warn`` or ``RUST_LOG=error`` in the
+environment::
+
+ $ RUST_LOG=warn ./target/release/bootsim run ...