Tag Browser as final to discourage inheriting from it

Author: WyriHaximus

README.md

@@ -1866,6 +1866,8 @@ $connector = new React\Socket\Connector($loop, array(
 $browser = new React\Http\Browser($loop, $connector);
 ```
 
+> Note that the browser class is final and shouldn't be extended, it is likely to be marked final in a future release.
+
 #### get()
 
 The `get(string $url, array $headers = array()): PromiseInterface<ResponseInterface>` method can be used to

src/Browser.php

@@ -13,6 +13,9 @@
 use React\Socket\ConnectorInterface;
 use React\Stream\ReadableStreamInterface;
 
+/**
+ * @final This class is final and shouldn't be extended as it is likely to be marked final in a future relase.
+ */
 class Browser
 {
     private $transaction;