Add metrics mode proposal to README

Author: xueweiz

README.md

@@ -187,3 +187,4 @@ Kubernetes cluster to a healthy state. The following remedy systems exist:
 * [Slides](https://docs.google.com/presentation/d/1bkJibjwWXy8YnB5fna6p-Ltiy-N5p01zUsA22wCNkXA/edit?usp=sharing)
 * [Plugin Interface Proposal](https://docs.google.com/document/d/1jK_5YloSYtboj-DtfjmYKxfNnUxCAvohLnsH5aGCAYQ/edit#)
 * [Addon Manifest](https://github.com/kubernetes/kubernetes/tree/master/cluster/addons/node-problem-detector)
+* [Metrics Mode Proposal](https://docs.google.com/document/d/1SeaUz6kBavI283Dq8GBpoEUDrHA2a795xtw0OvjM568/edit)