Add draft of Usage section.

README.rst

@@ -13,7 +13,9 @@ or "μ" in the name comes from the the SI prefix "micro-". uModbus is very small
 and lightweight. uModbus is open source and licenced under `Mozilla Public
 License`_. The source can be found on GitHub_.
 
-Routing Modbus requests is easy::
+Routing Modbus requests is easy:
+
+.. code:: python
 
     from umodbus import get_server
 

docs/source/usage.rst

@@ -0,0 +1,51 @@
+Usage
+-----
+
+Viewpoint
+=========
+
+uModbus is build with routing in mind. Routing (groups of) requests to a
+certain callback must be easy. This is in contrast with with other Modbus
+implementation which often focus on reading and writing from a data store.
+
+Because of this difference in view point uModbus doesn't know the concepts of
+Modbus' data models (discrete inputs, coils, input registers and holding
+registers) and their read/write properties.
+
+Routing
+-------
+
+The route system was inspired by Flask_. Like Flask, uModbus requires a global
+app or server. This server contains a route map. Routes can be added to this
+route map. The following example demostrates how to route all Modbus requests
+for slave 1, function code 1 en 2 addressed to address 0 to the endpoint
+`read_gpio_status`.
+
+.. code:: python
+
+    from umodbus import get_server
+    from collections import defaultdict
+
+    # A very simple data store which maps addresss against their values.
+    data_store = dict(int)
+
+    server = get_server('localhost', 502)
+
+    @server.route(slave_ids=[1], functions_codes=[3, 4], addresses=list(range(0, 10)))
+    def read_data_store(slave_id, address):
+        """" Return value of address. """
+        return data_store[address]
+
+    @server.route(slave_ids=[1], functions_codes=[3, 4], addresses=list(range(0, 10)))
+    def write_data_store(slave_id, address, value):
+        """" Set value for address. """
+        data_store[address] = value
+
+    try:
+        server.serve_forever()
+    finally:
+        server.stop()
+
+
+.. External references
+.. _Flask: http://flask.pocoo.org/