add doc

lib/src/thread_header.dart

@@ -2,9 +2,64 @@ import 'package:flutter/material.dart';
 import 'package:stream_chat/stream_chat.dart';
 import 'package:stream_chat_flutter/src/stream_chat_theme.dart';
 
+/// ![screenshot](https://raw.githubusercontent.com/GetStream/stream-chat-flutter/master/screenshots/thread_header.png)
+/// ![screenshot](https://raw.githubusercontent.com/GetStream/stream-chat-flutter/master/screenshots/thread_header_paint.png)
+///
+/// It shows the current thread information.
+///
+/// ```dart
+/// class ThreadPage extends StatelessWidget {
+///   final Message parent;
+///
+///   ThreadPage({
+///     Key key,
+///     this.parent,
+///   }) : super(key: key);
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return Scaffold(
+///       appBar: ThreadHeader(
+///         parent: parent,
+///       ),
+///       body: Column(
+///         children: <Widget>[
+///           Expanded(
+///             child: MessageListView(
+///               parentMessage: parent,
+///             ),
+///           ),
+///           MessageInput(
+///             parentMessage: parent,
+///           ),
+///         ],
+///       ),
+///     );
+///   }
+/// }
+/// ```
+///
+/// Usually you would use this widget as an [AppBar] inside a [Scaffold].
+/// However you can also use it as a normal widget.
+///
+/// Make sure to have a [StreamChannel] ancestor in order to provide the information about the channel.
+/// Every part of the widget uses a [StreamBuilder] to render the channel information as soon as it updates.
+///
+/// By default the widget shows a backButton that calls [Navigator.pop].
+/// You can disable this button using the [showBackButton] property of just override the behaviour
+/// with [onBackPressed].
+///
+/// The widget components render the ui based on the first ancestor of type [StreamChatTheme] and on its [ChannelTheme.channelHeaderTheme] property.
+/// Modify it to change the widget appearance.
 class ThreadHeader extends StatelessWidget implements PreferredSizeWidget {
+  /// True if this header shows the leading back button
   final bool showBackButton;
+
+  /// Callback to call when pressing the back button.
+  /// By default it calls [Navigator.pop]
   final VoidCallback onBackPressed;
+
+  /// The message parent of this thread
   final Message parent;
 
   ThreadHeader({