Add doc on validation of custom categories

Author: varshaprasad96

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

@@ -143,15 +143,17 @@ operatorhub    name=operatorhub           OperatorHub.io metadata validation
 ...
 ```
 
-For example, you want to turn on the `operatorhub` validator shown above so you can publish the `0.0.1` operator
-you recently created on [OperatorHub.io][operatorhub]. To do so, you can modify your Makefile's `bundle` recipe
-to validate any further changes you make to bundle UI metadata related to OperatorHub requirements:
+Documentation on optional validators:
+- [`operatorhub`][operatorhub_validator]
 
 ```make
 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
 
@@ -320,3 +322,4 @@ being managed, each with a `name` and `url`.
 [csv-markers]:/docs/building-operators/golang/references/markers
 [operatorhub]:https://operatorhub.io/
 [scorecard]:/docs/advanced-topics/scorecard
+[operatorhub_validator]: https://olm.operatorframework.io/docs/tasks/validate-package/#validation