docs: Add documentation about running on Openshift.

Signed-off-by: John Allberg <john@ayoy.se>

docs/docs/installation/index.md

@@ -195,6 +195,25 @@ To implement this:
 * Paste those lines into your `values.yaml` file
   and modify their values in that file.
 
+### Installing on Openshift
+
+To install on Openshift, set the value `global.isOpenshift` to true.
+In practice this means that runAsUser and runAsGroup is removed, since
+Openshift set those automatically.
+
+Setting `global.isOpenshift` can be done when running helm installation command:
+
+```shell
+helm install keptn keptn --set global.isOpenshift=true
+```
+
+or by including it in your values.yaml:
+
+```yaml
+global:
+  isOpenshift: true
+```
+
 ### Modify Helm values
 
 To modify Helm values: