Document programmatic tab selection

docs/_sidebar.md

   - [Top Level](/docs/top-level-api)
   - [Screen](/docs/screen-api)
   - [Events](/docs/events)
-  - [Layout Types](/docs/layout-types)
+  - [Layouts](/docs/layout-types)
   - [Styling](/docs/styling)
   - [TopBar Buttons](/docs/topBar-buttons) 
   - [Animations](/docs/animations)

docs/docs/layout-types.md

-# Layout Types
+# Layouts
 
 The possibilities of the RNN layout API are wide open in terms of what you can construct with it: stacks, tabs and drawers in many combinations.
 
 }
 ```
 
+### Selecting tabs programmatically
+
+The selected index is a style property which can be updated using the `mergeOptions` command. In order to update the BottomTabs options, Pass the BottomTabs `componentId` or the `componentId` of one of its children.
+
+?>We'll use the following BottomTabs layout to demonstrate programmatic tab selection.
+
+```js
+const bottomTabs = {
+  id: 'BottomTabsId',
+  children: [
+    {
+      component: {
+        name: 'FirstScreen',
+        options: { ... }
+      }
+    },
+    {
+      component: {
+        id: 'SecondScreenId',
+        name: 'SecondScreen',
+        options: { ... }
+      }
+    }
+  ]
+}
+```
+
+#### selecting a tab by index
+
+The following `mergeOptions` command will select the second tab. We're passing the id of our BottomTabs, but we could also use the id of any of the child components, for example `SecondScreenId`.
+
+```js
+Navigation.mergeOptions('BottomTabsId', {
+  bottomTabs: {
+    currentTabIndex: 1
+  }
+});
+```
+
+#### selecting a tab by componentId
+
+Tabs can also be selected by componentId. This is particularly useful in cases where you don't know in which tab a screen is contained.
+
+For example, if invoked from one of the child components;`SecondScreen` or `FirstScreen`, the following merge command will select the tab containing the child.
+
+```js
+Navigation.mergeOptions(this.props.componentId, {
+  bottomTabs: {
+    currentTabId: this.props.componentId
+  }
+});
+```
+
 ## sideMenu
 
 Expect center, left and right layouts
 }
 ```
 
-# Layout Examples
+## Layout Examples
 
-## Single page app with two side menus:
+### Single page app with two side menus:
 
 ```js
 Navigation.setRoot({
 });
 ```
 
-## Tab based app (with passProps example):
+### Tab based app (with passProps example):
 
 ```js
 Navigation.setRoot({
 });
 ```
 
-## Stack based app (with options example, initialised with 2 screens):
+### Stack based app (with options example, initialised with 2 screens):
 
 ```js
 Navigation.setRoot({