Update readme with more content

README.md

@@ -1,6 +1,19 @@
 # Knobs
 > A Prop Editor for react storybook
 
+![usage](usage.gif)
+
+## API
+
+Use function `createKnob` passed to your stories to create a *knob* for a prop.
+
+`createKnob(name, initialValue, type)`  
+
+* `name`: Should uniquely idintify the prop. Used as the label of the knob
+* `initialValue`: Value used when first rendered
+* `type`: Optionally specify a type for the prop. Decides the input type of the knob. Currently the string 'object' is identified and it gives knob a textarea input that will be `eval`ed for an object. By default shows a text input.
+
+
 ```js
 import React from 'react';
 import Button from './Button';