add docs

lib/src/full_screen_image.dart

@@ -2,9 +2,12 @@ import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 import 'package:photo_view/photo_view.dart';
 
+/// A full screen image widget
 class FullScreenImage extends StatelessWidget {
+  /// The url of the image
   final String url;
 
+  /// Instantiate a new FullScreenImage
   const FullScreenImage({
     Key key,
     @required this.url,