add documentation

Author: danielang

README.md

@@ -136,6 +136,24 @@ copilot({
 })(RootComponent)
 ```
 
+### Custom step number component
+You can customize the step number by passing a component to the `copilot` HOC maker. If you are looking for an example step number component, take a look at [the default step number implementation](https://github.com/okgrow/react-native-copilot/blob/master/src/components/StepNumber.js).
+
+```js
+const StepNumberComponent = ({
+  isFirstStep,
+  isLastStep,
+  currentStep,
+  currentStepNumber,
+}) => (
+  // ...
+);
+
+copilot({
+  stepNumberComponent: StepNumberComponent
+})(RootComponent)
+```
+
 ### Custom components as steps
 The components wrapped inside `CopilotStep`, will receive a `copilot` prop of type `Object` which the outermost rendered element of the component or the element that you want the tooltip be shown around, must extend.