add helm_extensions documentation

docs/source/admin_guide/custom-helm-charts.md

@@ -0,0 +1,22 @@
+# Other Helm Charts
+
+Arbitrary helm charts can be deployed and managed with the qhub-config.yaml file.
+
+Prefect is a workflow automation system. QHub integrates Prefect with a
+feature flag as follows (in the top level):
+
+As an example, deployinging the redis helm chart with qhub might look like the below:
+
+```yaml
+helm_extensions:
+  - name: my-redis-deployment    
+    repository: https://charts.bitnami.com/bitnami    
+    chart: redis    
+    version: 15.5.1
+    overrides:
+        diagnosticMode:
+            enabled: true
+```
+
+The `overrides` section is optional, but corresponds to the helm chart's [values.yaml](https://helm.sh/docs/chart_template_guide/values_files/) file, and allows you to override the default helm chart settings.
+

docs/source/admin_guide/index.md

@@ -13,5 +13,6 @@ system_maintenance.md
 monitoring.md
 clearml.md
 prefect.md
+custom-helm-charts.md
 faq.md
 ```