add doc

lib/src/channel_image.dart

@@ -3,6 +3,45 @@ import 'package:flutter/material.dart';
 import 'package:stream_chat/stream_chat.dart';
 import 'package:stream_chat_flutter/stream_chat_flutter.dart';
 
+/// ![screenshot](https://raw.githubusercontent.com/GetStream/stream-chat-flutter/master/screenshots/channel_image.png)
+/// ![screenshot](https://raw.githubusercontent.com/GetStream/stream-chat-flutter/master/screenshots/channel_image_paint.png)
+///
+/// It shows the current [Channel] image.
+///
+/// ```dart
+/// class MyApp extends StatelessWidget {
+///   final Client client;
+///   final Channel channel;
+///
+///   MyApp(this.client, this.channel);
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return MaterialApp(
+///       debugShowCheckedModeBanner: false,
+///       home: StreamChat(
+///         client: client,
+///         child: StreamChannel(
+///           channel: channel,
+///           child: Center(
+///             child: ChannelImage(
+///               channel: channel,
+///             ),
+///           ),
+///         ),
+///       ),
+///     );
+///   }
+/// }
+/// ```
+///
+/// The widget uses a [StreamBuilder] to render the channel information image as soon as it updates.
+///
+/// By default the widget radius size is 40x40 pixels.
+/// Set the property [size] to set a custom dimension.
+///
+/// The widget renders the ui based on the first ancestor of type [StreamChatTheme].
+/// Modify it to change the widget appearance.
 class ChannelImage extends StatelessWidget {
   const ChannelImage({
     Key key,
@@ -10,7 +49,10 @@ class ChannelImage extends StatelessWidget {
     this.size = 40,
   }) : super(key: key);
 
+  /// The channel to show the image of
   final Channel channel;
+
+  /// The diameter of the image
   final double size;
 
   @override