add setTabButton instructions (#1195)

docs/screen-api.md

   badge: 17 // badge value, null to remove badge
 });
 ```
+## setTabButton(params = {})
+
+Change the tab icon on a bottom tab.
+
+```js
+this.props.navigator.setTabButton({
+  tabIndex: 0, // (optional) if missing, the badge will be added to this screen's tab
+  icon: require('../img/one.png'), // local image asset for the tab icon unselected state (optional)
+  selectedIcon: require('../img/one_selected.png'), // local image asset for the tab icon selected state (optional, iOS only)
+});
+```
 
 ## switchToTab(params = {})