Added custom resource spec for ArangoDB Storage operator.

docs/user/storage_resource.md

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+# ArangoLocalStorage Custom Resource
+
+The ArangoDB Storage operator creates and maintains ArangoDB
+storage resources in a Kubernetes cluster, given a storage specification.
+This storage specification is a CustomResource following
+a CustomResourceDefinition created by the operator.
+
+Example minimal storage definition:
+
+```yaml
+apiVersion: "storage.arangodb.com/v1alpha"
+kind: "ArangoLocalStorage"
+metadata:
+ name: "example-arangodb-storage"
+spec:
+ storageClass:
+ name: my-local-ssd
+ localPath:
+ - /mnt/big-ssd-disk
+```
+
+This definition results in:
+
+- a `StorageClass` called `my-local-ssd`
+- the dynamic provisioning of PersistentVolume's with
+ a local volume on a node where the local volume starts
+ in a sub-directory of `/mnt/big-ssd-disk`.
+- the dynamic cleanup of PersistentVolume's (created by
+ the operator) after one is released.
+
+## Specification reference
+
+Below you'll find all settings of the `ArangoLocalStorage` custom resource.
+
+### `spec.storageClass.name: string`
+
+This setting specifies the name of the storage class that
+creates `PersistentVolume` will use.
+
+If empty, this field defaults to the name of the `ArangoLocalStorage`
+object.
+
+If a `StorageClass` with given name does not yet exist, it
+will be created.
+
+### `spec.storageClass.isDefault: bool`
+
+This setting specifies if the created `StorageClass` will
+be marked as default storage class. (default is `false`)
+
+### `spec.localPath: stringList`
+
+This setting specifies one of more local directories
+(on the nodes) used to create persistent volumes in.
+
+### `spec.nodeSelector: nodeSelector`
+
+This setting specifies which nodes the operator will
+provision persistent volumes on.