Update community supported README: legal disclaimer (#227)

Author: cwarnermm

README.md

@@ -1,3 +1,7 @@
+# Disclaimer
+
+**This repository is community supported and not maintained by Mattermost. Mattermost disclaims liability for integrations, including Third Party Integrations and Mattermost Integrations. Integrations may be modified or discontinued at any time.**
+
 # Mattermost Todo Plugin
 
 [![Build Status](https://img.shields.io/circleci/project/github/mattermost/mattermost-plugin-todo/master.svg)](https://circleci.com/gh/mattermost/mattermost-plugin-todo)