feature #12598 Update documentation for MercureBundle: add enable_profiler option (vincentchalamon)

This PR was submitted for the master branch but it was squashed and merged into the 4.3 branch instead (closes #12598).

Discussion
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Update documentation for MercureBundle: add enable_profiler option

Related feature: symfony/mercure-bundle#13

![panel](https://user-images.githubusercontent.com/995707/68297999-9d299b00-0098-11ea-9662-86062da00bf8.png)

Commits
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3705dae Update documentation for MercureBundle: add enable_profiler option

Author: OskarStark

_images/mercure/panel.png

@@ -568,6 +568,44 @@ sent. Here is the PublisherStub implementation::
     App\Tests\Functional\Fixtures\PublisherStub:
         decorates: mercure.hub.default.publisher
 
+.. versionadded:: 0.2
+
+Debugging
+---------
+
+The WebProfiler panel was introduced in MercureBundle 0.2. Enable it on your configuration, as following:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/packages/mercure.yaml
+        mercure:
+            enable_profiler: '%kernel.debug%'
+
+    .. code-block:: xml
+
+        <!-- config/packages/mercure.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                https://symfony.com/schema/dic/services/services-1.0.xsd">
+
+            <mercure:config enable_profiler="%kernel.debug%"/>
+
+        </container>
+
+    .. code-block:: php
+
+        // config/packages/mercure.php
+        $container->loadFromExtension('mercure', [
+            'enable_profiler' => '%kernel.debug%',
+        ]);
+
+
+.. image:: /_images/mercure/panel.png
+
 .. _`the Mercure protocol`: https://github.com/dunglas/mercure#protocol-specification
 .. _`Server-Sent Events (SSE)`: https://developer.mozilla.org/docs/Server-sent_events
 .. _`a polyfill`: https://github.com/Yaffle/EventSource