docs: add glasskube as an alternative installation method

Signed-off-by: Thomas Schuetz <[email protected]>

docs/docs/installation/index.md

@@ -121,6 +121,19 @@ Some helpful hints:
 
     The output shows all Keptn components that are running on your cluster.
 
+### Alternative installation methods
+
+#### Install Keptn via Glasskube
+Glasskube is a package manager for Keptn that simplifies the installation process and can be used to automatically upgrade Keptn to the latest version. For more information how to install Glasskube, see [Glasskube Installation](https://glasskube.dev/docs/getting-started/install/).
+
+To install Keptn via Glasskube after Glasskube is installed, run the following command:
+
+``` shell
+glasskube install keptn
+```
+
+After that, keptn is installed on your cluster and will be automatically upgraded to the latest version.
+
 ## Keptn Helm configuration
 
 The Keptn configuration is controlled by a set of Helm value files,