Document StorageClass override (#441)

* draft

* removed whitespace

* More description

* More description

* typos

* changed some formulation

Author: fhennig

modules/concepts/pages/resources.adoc

@@ -98,3 +98,23 @@ In this case, the role group `resources-from-role` will inherit the resources sp
 The role group `resources-from-role-group` has maximum of `4Gi` storage space for `data` (which overrides the role resources).
 
 IMPORTANT: Stackable operators use different names (`data` in this example) for their storage specification. This is documented in the operator specific documentation.
+
+[#storageclass]
+==== StorageClass
+
+A StorageClass defines a type of storage with certain properties.
+The StorageClasses that are available on a Kubernetes cluster are configured by the cluster administrator.
+Different classes can be configured to provide different levels of reliability or speed, or be configured to be more suited for read or write heavy loads.
+This configuration is either done in the storage backend or Kubernetes settings (find more information in the https://kubernetes.io/docs/concepts/storage/storage-classes/[Kubernetes documentation]).
+
+For Stackable resources, setting a StorageClass is not mandatory; if not StorageClass is set, the https://kubernetes.io/docs/concepts/storage/storage-classes/#default-storageclass[default StorageClass] will be used. If you want to use a specific StorageClass for a particular storage, the StorageClass can be set on the resource:
+
+[source,yaml]
+----
+...
+resources:
+  storage:
+    data: # name of the storage
+      capacity: 4Gi
+      storageClass: my-storage-class
+----