Add screenshots to flutter_simple_treeview (#432)

Author: szakarias

packages/flutter_simple_treeview/CHANGELOG.md

@@ -1,3 +1,7 @@
+# 3.0.2
+
+  * Add screenshots
+
 # 3.0.1
 
   * Minor fixes.

packages/flutter_simple_treeview/example/screenshots/treeview_demo.png

@@ -1,5 +1,5 @@
 name: flutter_simple_treeview
-version: 3.0.1
+version: 3.0.2
 description: >
   A widget, that visualises a tree structure, where a node can be any widget.
 repository: https://github.com/google/flutter.widgets/tree/master/packages/flutter_simple_treeview
@@ -16,3 +16,9 @@ dev_dependencies:
   test:
   flutter_test:
     sdk: flutter
+
+screenshots:
+  - description: 'Example of tree controller usage.'
+    path: example/screenshots/treeview_demo.png
+  - description: 'Example of tree from JSON.'
+    path: example/screenshots/treeview_demo_JSON.png