samples: Added README for watchdog sample

Added README for watchdog sample in rst format

Signed-off-by: Alejandro Hidalgo Muñoz de Rivera <ahmrivera@gmail.com>

samples/drivers/watchdog/README.rst

@@ -0,0 +1,55 @@
+.. _watchdog-sample:
+
+Watchdog Sample
+###############
+
+Overview
+********
+
+This sample demonstrates how to use the watchdog driver API.
+
+A typical use case for a watchdog is that the board is restarted in case some piece of code
+is kept in an infinite loop.
+
+Building and Running
+********************
+
+In this sample, a watchdog callback is used to handle a timeout event once. This functionality is used to request an action before the board
+restarts due to a timeout event in the watchdog driver.
+
+The watchdog peripheral is configured in the board's ``.dts`` file. Make sure that the watchdog is enabled
+using the configuration file in ``boards`` folder.
+
+Building and Running for ST Nucleo F091RC
+=========================================
+
+The sample can be built and executed for the
+:ref:`nucleo_f091rc_board` as follows:
+
+.. zephyr-app-commands::
+	:zephyr-app: samples/drivers/watchdog
+	:board: nucleo_f091rc
+	:goals: build flash
+	:compact:
+
+To build for another board, change "nucleo_f091rc" to the name of that board and provide a corresponding devicetree overlay.
+
+Sample output
+=============
+
+You should get a similar output as below:
+
+.. code-block:: console
+
+	Watchdog sample application
+	Attempting to test pre-reset callback
+	Feeding watchdog 5 times
+	Feeding watchdog...
+	Feeding watchdog...
+	Feeding watchdog...
+	Feeding watchdog...
+	Feeding watchdog...
+	Waiting for reset...
+	Handled things..ready to reset
+
+.. note:: After the last message, the board will reset and the sequence will start again