README: update information on design principles

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

README

@@ -1,5 +1,25 @@
 UPM - Sensor/Actuator repository for Maa
 
-UPM is a high level repository for sensors that use maa.
+UPM is a high level repository for sensors that use maa. Each sensor links to
+libmaa and are not meant to be interlinked although some groups of sensors may
+be. Each sensor contains a header which allows to interface with it. Typically
+a sensor is represented as a class and instanciated.
+
+The constructor is expected to initialise the sensor and paramters may be used
+to provide identification/pin location on the board.
+
+Typically an update() function will be called in order to get new data from the
+sensor in order to reduce load when doing multiple reads to sensor data.
+
+A basic sensor is expected to work as such:
+s = new sensor();
+print(sensor->read());
+sleep(1);
+s->update();
+print(sensor->read();
+
+However implementation and API design is compeltely up to the developer, some
+enumerable sensors for example may provide much clever instanciation. Displays
+may also create more complex structures in order to interface with them.
 
 For more information on maa, see the maa README