samples: doc: Added README for shell devmem_load sample

samples/subsys/shell/devmem_load/README.rst

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+.. zephyr:code-sample:: shell-devmem-load
+   :name: devmem load shell
+   :relevant-api: shell_api
+
+   Load data to device memory using devmem shell cmd.
+
+Overview
+********
+
+This module adds a ``devmem load`` command that allows data to be loaded into
+device memory. The ``devmem load`` command is supported by every transport the
+Zephyr shell can run on.
+
+After invoking the command in the Zephyr shell, the device receives the
+transferred data and writes it to the specified memory address.
+The transfer ends when the user presses :kbd:`Ctrl+X` followed by :kbd:`Ctrl+Q`.
+
+Requirements
+************
+
+* A target configured with the shell interface, exposed through any of
+  its :ref:`backends <backends>`.
+
+Building and Running
+********************
+
+The sample can be built for several platforms.
+
+Emulation Targets
+=================
+
+The sample may run on emulation targets. The following commands build the
+application for the qemu_x86.
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/shell/devmem_load
+   :host-os: unix
+   :board: qemu_x86
+   :goals: run
+   :compact:
+
+After running the application, the console displays the shell interface, and
+shows the shell prompt, at which point the user may type in the ``devmem load`` command.
+
+.. note::
+
+   When using the ``devmem load`` command over UART, it is recommended to use
+   interrupts whenever possible.
+   If this is not possible, reduce the baud rate to 9600.
+
+   If you use polling mode, you should also use ``prj_poll.conf`` instead of
+   ``prj.conf``.
+
+Building for boards without UART interrupt support:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/shell/devmem_load
+   :host-os: unix
+   :board: native_sim
+   :gen-args: -DCONF_FILE=prj_poll.conf
+   :goals: run
+   :compact:
+
+On-Hardware
+===========
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/shell/shell_module
+   :host-os: unix
+   :board: nrf52840dk/nrf52840
+   :goals: flash
+   :compact:
+
+Sample Output
+*************
+
+After connecting to the UART console, you should see the following output:
+
+.. code-block:: console
+
+   uart:~$
+
+The ``devmem load`` command can now be used:
+
+.. code-block:: console
+
+   uart:~$ devmem load 0x20020000
+   Loading...
+   press ctrl-x ctrl-q to escape
+
+At this point, the ``devmem load`` command waits for data input.
+You can either type the data directly from the console or send it from the host PC
+(replace ``ttyX`` with your UART console device):
+
+.. code-block:: bash
+
+   xxd -p data > /dev/ttyX
+
+.. note::
+
+   It is important to use a plain-style hex dump.
+
+Once the data transfer is complete, use :kbd:`Ctrl+X` followed by :kbd:`Ctrl+Q`
+to quit the loader. The shell then displays the number of bytes read and returns
+to the prompt:
+
+.. code-block:: console
+
+   Number of bytes read: 3442
+   uart:~$
+
+Options
+========
+
+The ``devmem load`` command currently supports the following option:
+
+* ``-e``
+  Interpret data as little endian, e.g. ``0xDEADBEFF`` → ``0xFFBEADDE``.