Add documentation of NORMALIZER_CLASS

Author: jpfinne

docs/customization.md

@@ -601,6 +601,13 @@ Example:
 java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g java -i modules/openapi-generator/src/test/resources/3_0/enableKeepOnlyFirstTagInOperation_test.yaml -o /tmp/java-okhttp/ --openapi-normalizer REMOVE_X_INTERNAL=true
 ```
 
+- `NORMALIZER_CLASS`: Set to full classname of a class extending the default org.openapitools.codegen.OpenAPINormalizer. It allows customization of the default normalizer. 
+
+Example:
+```
+java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g java -i modules/openapi-generator/src/test/resources/3_0/required-properties.yaml -o /tmp/java-okhttp/ --openapi-normalizer NORMALIZER_CLASS=org.openapitools.codegen.OpenAPINormalizerTest$RemoveRequiredNormalizer
+```
+
 - `FILTER`
 
 The `FILTER` parameter allows selective inclusion of API operations based on specific criteria. It applies the `x-internal: true` property to operations that do **not** match the specified values, preventing them from being generated.