doc: add Modbus RTU documentation

Add Modbus RTU documentation.

Signed-off-by: Johann Fischer <j.fischer@phytec.de>

doc/reference/index.rst

@@ -23,6 +23,7 @@ API Reference
    logging/index.rst
    misc/index
    misc/data_structures.rst
+   modbus/index.rst
    networking/index.rst
    peripherals/index.rst
    power_management/index.rst

doc/reference/modbus/index.rst

@@ -0,0 +1,9 @@
+.. _modbus:
+
+MODBUS
+######
+
+.. toctree::
+   :maxdepth: 2
+
+   modbus.rst

doc/reference/modbus/modbus.rst

@@ -0,0 +1,36 @@
+.. _modbus_api:
+
+MODBUS over serial line (RTU)
+#############################
+
+Modbus is an industrial messaging protocol. The protocol is specified
+for different types of networks or buses. Zephyr RTOS implementation
+is currently only available for serial line and may be used
+with different physical interfaces, like RS485 or RS232.
+
+Modbus communication via serial line is based on client/server model.
+Only one client may be present on the bus. Client can communicate with several
+server devices. Server devices themselves are passive and must not send
+requests or unsolicited responses.
+Services requested by the client are specified by function codes (FCxx),
+and can be found in the specification or documentation of the API below.
+
+Zephyr RTOS implementation supports both client and server roles.
+
+More information about Modbus and Modbus RTU can be found on the website
+`MODBUS Protocol Specifications`_.
+
+Samples
+*******
+
+`modbus-rtu-server-sample` and `modbus-rtu-client-sample` give
+the possibility to try out RTU server and RTU client implementation with
+an evaluation board.
+
+API Reference
+*************
+
+.. doxygengroup:: modbus
+   :project: Zephyr
+
+.. _`MODBUS Protocol Specifications`: https://www.modbus.org/specs.php