fixes #242 add customized fetcher and meta schema doc

Author: stevehu

README.md

@@ -104,6 +104,13 @@ For the latest version, please check the [release](https://github.com/networknt/
 
 ## [YAML Validation](doc/yaml.md)
 
+## [Schema Mapping](doc/schema-map.md)
+
+## [Customized URIFetcher](doc/cust-fetcher.md)
+
+## [Customized MetaSchema](doc/cust-meta.md)
+
+
 ## Known issues
 
 I have just updated the test suites from the [official website](https://github.com/json-schema-org/JSON-Schema-Test-Suite) as the old ones were copied from another Java validator. Now there are several issues that need to be addressed. All of them are edge cases, in my opinion, but need to be investigated. As my old test suites were inherited from another Java JSON Schema Validator, I guess other Java Validator would have the same issues as these issues are in the Java language itself.

doc/cust_fetcher.md

@@ -0,0 +1 @@
+The default URIFetcher implementation uses JDK connection/socket without handling network exceptions. It works in most of the cases; however, if you want to have a customized implementation, you can do so. One user has his implementation with urirest to handle the timeout. A detailed discussion can be found in this [issue](https://github.com/networknt/json-schema-validator/issues/240)

doc/cust_meta.md

@@ -0,0 +1,6 @@
+How to use your customized meta schema?
+
+The library supports customized meta schema with this builder method. We have provided default instances for v4, v6, v7, and v2019-09 but users can always create their own JsonMetaSchema instance with customized meta schema.
+
+https://github.com/networknt/json-schema-validator/blob/master/src/main/java/com/networknt/schema/JsonMetaSchema.java#L188
+