add a doc for metaschema validation

Author: stevehu

README.md

@@ -127,6 +127,8 @@ For the latest version, please check the [release](https://github.com/networknt/
 
 ## [Multiple Language](doc/multiple-language.md)
 
+## [MetaSchema Validation](doc/metaschema-validation.md)
+
 
 ## Known issues
 

doc/metaschema-validation.md

@@ -0,0 +1,20 @@
+If you have an use case to validate custom schemas against the one of the JSON schema draft version, here is the code that you can do it. 
+
+```
+  public static final Function<ObjectNode, Set<SchemaValidationMessage>> validateAgainstMetaSchema =
+      schema -> {
+        JsonSchemaFactory factory = JsonSchemaFactory.getInstance(SpecVersion.VersionFlag.V201909);
+         JsonSchema metaSchema = factory.getSchema(getSchemaUri());
+        return metaSchema.validate(schema).stream()
+            .map((validation) -> new SchemaValidationMessage(validation.getMessage()))
+            .collect(Collectors.toSet());
+      };
+
+```
+
+This should now work but does not support all the keywords because the JsonMetaSchema of SpecVersion.VersionFlag.V201909 is lacking these features.
+
+You can fix the issue by resolving the vocabularies to a local resource file and re-do the JsonMetaSchema for 2019 based on that. 
+
+
+