Add content-type plugin documentation

plugins/content-type.rst

@@ -0,0 +1,18 @@
+Content-Type Plugin
+===================
+
+The ``ContentTypePlugin`` sets the correct ``Content-Type`` header value based on the content of the body stream of the
+request. This helps HTTP servers to handle the request::
+
+    use Http\Discovery\HttpClientDiscovery;
+    use Http\Client\Common\PluginClient;
+    use Http\Client\Common\Plugin\ContentTypePlugin;
+
+    $contentTypePlugin = new ContentTypePlugin();
+
+    $pluginClient = new PluginClient(
+        HttpClientDiscovery::find(),
+        [$contentTypePlugin]
+    );
+
+For now, the plugin can only detect JSON or XML content. If the content of the stream can not be determined, the plugin does nothing.

plugins/index.rst

@@ -13,6 +13,7 @@ request or you can even start a completely new request. This gives you full cont
    authentication
    cache
    content-length
+   content-type
    cookie
    decoder
    error