Added plugins to documentation
# Summary
* [Introduction](README.md)
-* [Installation](README.md)
+* Installation
* [Docker](install/docker.md)
-* [Quick Start](quick_start.md)
+* Quick Start
* [Auth](quick_start/auth.md)
* [File System](quick_start/file_system.md)
* [MongoDB](quick_start/mongodb.md)
* [Request HTTP method](proxy/request_http_method.md)
* [Routing priorities](proxy/routing_priorities.md)
* [Conclusion](proxy/conclusion.md)
+* Plugins
+ * [OAuth](plugins/oauth.md)
+ * [Rate Limit](plugins/rate_limit.md)
+ * [CORS](plugins/cors.md)
* Misc
* [Monitoring](misc/monitoring.md)
