Add doc on validation of custom categories

Author: varshaprasad96

website/content/en/docs/olm-integration/generation.md

@@ -143,6 +143,9 @@ bundle: ...
   ...
   operator-sdk bundle validate ./bundle --select-optional name=operatorhub
 ```
+The `operatorhub` validator also allows validation of custom `categories` in CSV [annotations][csv_annotation]. This can be done by
+defining a list of custom categories in a json-encoded file and setting its path as an environment variable named `OPERATOR_BUNDLE_CATEGORIES`. For more
+details refer to [validation of operator bundles][olm_validation].
 
 ### Package manifests format
 
@@ -310,3 +313,5 @@ being managed, each with a `name` and `url`.
 [olm-capabilities]:/docs/advanced-topics/operator-capabilities/operator-capabilities
 [csv-markers]:/docs/building-operators/golang/references/markers
 [operatorhub]:https://operatorhub.io/
+[olm_validation]: https://olm.operatorframework.io/docs/tasks/validate-package/#validation
+[csv_annotation]: https://github.com/kubernetes/apimachinery/blob/v0.19.3/pkg/apis/meta/v1/types.go#L238