(doc): Add vendors annotation as a prerequisite in contribution guide

 - vendors.odf.openshift.io/kind is required for displaying the external vendor in ODF UI

Signed-off-by: Afreen Rahman <afrahman@redhat.com>

frontend/packages/ceph-storage-plugin/src/components/create-storage-system/external-storage/README.md

@@ -5,6 +5,10 @@ OpenShift Data Foundation (ODF) operator supports connecting to an external vend
 
 All external vendors can contribute their UI via the `Create storage class` step of `CreateStorageSystem` wizard.
 
+#### Prerequisite:
+
+The `kind` of vendor should be present in [ODF operator CSV annotation](https://github.com/red-hat-storage/odf-operator/blob/405c2af1266fa11b53441dc3d4a329d330c999f5/config/manifests/bases/odf-operator.clusterserviceversion.yaml#L15) (`vendors.odf.openshift.io/kind`)
+
 Follow the steps written below to integrate with the ODF wizard: