docs(readme): add details for collapse and expand event listeners

Author: Zhivka Dimova

README.md

@@ -13,12 +13,14 @@ ng2-tree is a simple [Angular 2](https://github.com/angular/angular) component f
     - [Configure node via TreeModelSettings](#configure-node-via-treemodelsettings)
   - [[settings]](#settings)
   - [`Tree` class](#tree-class)
-  - [events (nodeMoved, nodeSelected, nodeRenamed, nodeRemoved, nodeCreated)](#events-nodemoved-nodeselected-noderenamed-noderemoved-nodecreated)
+  - [events (nodeMoved, nodeSelected, nodeRenamed, nodeRemoved, nodeCreated, nodeExpanded, nodeCollapsed)](#events-nodemoved-nodeselected-noderenamed-noderemoved-nodecreated-nodeexpanded-nodecollapsed)
     - [NodeSelectedEvent](#nodeselectedevent)
     - [NodeMovedEvent](#nodemovedevent)
     - [NodeRemovedEvent](#noderemovedevent)
     - [NodeCreatedEvent](#nodecreatedevent)
     - [NodeRenamedEvent](#noderenamedevent)
+    - [NodeExpandedEvent](#nodeexpandedevent)
+    - [NodeCollapsedEvent](#nodecollapsedevent)
 - [Changes that should be taken into account in order to migrate from __ng2-tree V1__ to __ng2-tree V2__](#changes-that-should-be-taken-into-account-in-order-to-migrate-from-__ng2-tree-v1__-to-__ng2-tree-v2__)
 - [:bulb: Want to help?](#bulb-want-to-help)
 
@@ -124,7 +126,9 @@ Here is the fully stuffed *tree* tag that you can use in your templates:
       (nodeRenamed)="handleRenamed($event)"
       (nodeSelected)="handleSelected($event)"
       (nodeMoved)="handleMoved($event)"
-      (nodeCreated)="handleCreated($event)">
+      (nodeCreated)="handleCreated($event)"
+      (nodeExpanded)="handleExpanded($event)"
+      (nodeCollapsed)="handleCollapsed($event)">
     </tree>
 ```
 
@@ -300,7 +304,7 @@ By default `rootIsVisible` equals to `true`
 
 Also in the next section you'll be reading about events generated by the `ng2-tree`. And here [Tree](src/tree.ts) class comes in handy for us, because its instances propagated with event objects. Under the hood `ng2-tree` wraps a `TreeModel` provided by the user in `Tree`. And `Tree` in turn has lots of useful methods and properties (like `parent`, `hasChild()`, `isRoot()` etc.)
 
-### events (nodeMoved, nodeSelected, nodeRenamed, nodeRemoved, nodeCreated)
+### events (nodeMoved, nodeSelected, nodeRenamed, nodeRemoved, nodeCreated, nodeExpanded, nodeCollapsed)
 
 Here is the diagram that shows tree events' hierarchy
 
@@ -406,6 +410,40 @@ You can subscribe to `NodeRenamedEvent` by attaching listener to `(nodeRenamed)`
 }
 ```
 
+#### NodeExpandedEvent
+
+You can subscribe to `NodeExpandedEvent` by attaching listener to `(nodeExpanded)` attribute, this event wont fire on initial expansion
+
+```html
+    <tree
+      [tree]="tree"
+      (nodeExpanded)="handleExpanded($event)">
+    </tree>
+```
+
+`NodeExpandedEvent` has a `node` property of type `Tree`, which contains an expanded node.
+
+```typescript
+{node: <Tree>{...}}
+```
+
+#### NodeCollapsedEvent
+
+You can subscribe to `NodeCollapsedEvent` by attaching listener to `(nodeCollapsed)` attribute
+
+```html
+    <tree
+      [tree]="tree"
+      (nodeCollapsed)="handleCollapsed($event)">
+    </tree>
+```
+
+`NodeCollapsedEvent` has a `node` property of type `Tree`, which contains a collapsed node.
+
+```typescript
+{node: <Tree>{...}}
+```
+
 ## Changes that should be taken into account in order to migrate from __ng2-tree V1__ to __ng2-tree V2__
 - Events were reworked:
   - In V1 all events that were inherited from NodeDestructiveEvent used to have property `parent`. It's not the case anymore. If you need a parent you should get it from `node` in event object like `node.parent`;