Unmaintained note in README (#643)

Repo is no longer maintained.

Author: adolsalamanca

README.md

@@ -1,3 +1,6 @@
+|![](https://upload.wikimedia.org/wikipedia/commons/thumb/1/17/Warning.svg/156px-Warning.svg.png) | This project is no longer maintained.
+|---|---|
+
 [![Test Helm Charts](https://github.com/timescale/helm-charts/actions/workflows/tests.yml/badge.svg)](https://github.com/timescale/helm-charts/actions/workflows/tests.yml)
 [![Commit activity](https://img.shields.io/github/commit-activity/m/timescale/helm-charts)](https://github.com/timescale/helm-charts/pulse/monthly)
 [![License](https://img.shields.io/github/license/timescale/helm-charts)](https://github.com/timescale/helm-charts/blob/main/LICENSE)