readme: Update readme to include cert-manager plugin

Signed-off-by: yolossn <[email protected]>

Author: yolossn

README.md

@@ -13,7 +13,7 @@ This is a repository of official plugins that Headlamp uses or recommends.
 | [opencost](./opencost)             | See the costs of your workloads in Headlamp.                                 |
 | [plugin-catalog](./plugin-catalog) | Install Headlamp plugins with one click.                                     | Shipped with Headlmp desktop builds by default.                                                                             |
 | [prometheus](./prometheus)         | Provides a Prometheus powered chart in the details views of workloads.       | Needs Prometheus installed in the cluster for the chart to be shown. Shipped with Headlmp desktop and CI builds by default. |
-
+| [cert-manager](./cert-manager)     | A UI for viewing and managing cert-manager.                                  | 
 ## Development
 
 For examples of plugins created for learning and documentation purposes, please refer to the [examples folder](https://github.com/headlamp-k8s/headlamp/tree/main/plugins#plugins) in Headlamp's repository.