update README with usage instructions

README.md

@@ -16,6 +16,14 @@ Other dependencies can be installed running:
 
 The application uses [bit.ly](https://bitly.com/) to shorten the urls and check they are from the BBC. This service require a bit.ly api key and account which can be got for free from the bit.ly website. Once you have these credentials the app expects them to live in a file called .bitlyrc in your home directory.
 
+## Usage
+
+Once the app is running, go to the url:
+
+    http://app.location/300?url=http://www.bbc.co.uk
+
+Where 300 is the width in pixels you would like the qrcode to be and the url to be encoded.
+
 ## Resources
 
 * [More fun with QRCodes and the BBC logo](http://whomwah.com/2008/03/12/more-fun-with-qr-codes-and-the-bbc-logo/)