Update README.rst to include usage example for periodic data acquisition.

Author: WoefulDerelict

README.rst

@@ -63,6 +63,33 @@ And then you can start measuring the temperature and humidity:
     print(sensor.temperature)
     print(sensor.relative_humidity)
 
+You can instruct the sensor to start periodically measuring the temperature
+and humidity at a set interval and retrieve the stored measurements:
+
+.. code:: python
+
+    sensor.mode = 'Periodic'
+    print(sensor.temperature)
+    print(sensor.relative_humidity)
+
+You can adjust the frequency at which the sensor periodically gathers data to:
+0.5, 1, 2, 4 or 10 Hz. The following adjusts the frequency to 2 Hz:
+
+.. code:: python
+
+    sensor.frequency = 2
+
+
+The sensor is capable of storing eight results. The sensor stores these
+results in an internal FILO cache.
+
+The sensor will continue to collect data at the set interval until it is
+returned to single shot data acquisition mode:
+
+.. code:: python
+
+    sensor.mode = 'Single'
+
 Contributing
 ============