Added docs about History plugin

Author: Nyholm

README.md

@@ -51,14 +51,17 @@ For information how to write applications with the services provided by this bun
 | httplug.stream_factory | Service* that provides the `Http\Message\StreamFactory`
 | httplug.client.[name] | This is your Httpclient that you have configured. With the configuration below the name would be `acme_client`.
 | httplug.client | This is the first client configured or a client named `default`.
-| httplug.plugin.content_length <br> httplug.plugin.decoder<br> httplug.plugin.error<br> httplug.plugin.logger<br> httplug.plugin.redirect<br> httplug.plugin.retry | These are build in plugins that lives in the `php-http/plugins` package. These servcies are not public and may only be used when configure HttpClients or services. 
+| httplug.plugin.content_length <br> httplug.plugin.decoder<br> httplug.plugin.error<br> httplug.plugin.history**<br> httplug.plugin.logger<br> httplug.plugin.redirect<br> httplug.plugin.retry | These are build in plugins that lives in the `php-http/plugins` package. These servcies are not public and may only be used when configure HttpClients or services. 
 
 \* *These services are always an alias to another service. You can specify your own service or leave the default, which is the same name with `.default` appended. The default services in turn use the service discovery mechanism to provide the best available implementation. You can specify a class for each of the default services to use instead of discovery, as long as those classes can be instantiated without arguments.*
 
+\** *The History plugin is only available when the kernel runs on debug mode and `httplug.toolbar: true`.*
+
 If you need a more custom setup, define the services in your application configuration and specify your service in the `main_alias` section. For example, to add authentication headers, you could define a service that decorates the service `httplug.client.default` with a plugin that injects the authentication headers into the request and configure `httplug.main_alias.client` to the name of your service.
 
 ```yaml
 httplug:
+  toolbar: true # Default
   clients: 
       acme_client: # This is the name of the client
         factory: 'httplug.factory.guzzle6'