Merge pull request #277 from francesc79/bug/cyclic-dep

add small documentation about URNFactory

Author: stevehu

doc/schema-map.md

@@ -17,3 +17,8 @@ Basically, you can specify a mapping in the builder. For more details, please ta
 ### Real Example
 
 https://github.com/JMRI/JMRI/blob/master/java/src/jmri/server/json/schema-map.json
+
+In case you provide the schema through an InputStream or a String to resolve $ref with URN (relative path), you need to provide the URNFactory to the JsonSchemaFactory.Builder.
+URNFactory interface will allow you to resolve URN to URI.
+
+please take a look at the test cases and the [PR](https://github.com/networknt/json-schema-validator/pull/274).