add readme test

Author: ladyada

README.rst

@@ -99,8 +99,26 @@ Or the following command to update an existing version:
 Usage Example
 =============
 
-.. todo:: Add a quick, simple example. It and other examples should live in the
-examples folder and be included in docs/examples.rst.
+.. code-block:: python
+
+    import time
+    import board
+    import adafruit_scd4x
+
+    i2c = board.I2C()
+    scd4x = adafruit_scd4x.SCD4X(i2c)
+    print("Serial number:", [hex(i) for i in scd4x.serial_number])
+
+    scd4x.start_periodic_measurement()
+    print("Waiting for first measurement....")
+
+    while True:
+        if scd4x.data_ready:
+            print("CO2: %d ppm" % scd4x.CO2)
+            print("Temperature: %0.1f *C" % scd4x.temperature)
+            print("Humidity: %0.1f %%" % scd4x.relative_humidity)
+            print()
+        time.sleep(1)
 
 Contributing
 ============