fix(docs): fix invalid repo links in README (apache#505)

Signed-off-by: Haoran Yu <[email protected]>

README.md

@@ -231,8 +231,8 @@ for details about the PySpark library.
 
 ## Contributing
 
-- Start with [Contributing Guide]().
-- Submit [Issues]() for bug reports, feature requests.
+- Start with [Contributing Guide](https://github.com/apache/incubator-graphar/blob/main/CONTRIBUTING.md).
+- Submit [Issues](https://github.com/apache/incubator-graphar/issues) for bug reports, feature requests.
 - Discuss at [dev mailing list](mailto:[email protected]) ([subscribe](mailto:[email protected]?subject=(send%20this%20email%20to%20subscribe)) / [unsubscribe](mailto:[email protected]?subject=(send%20this%20email%20to%20unsubscribe)) / [archives](https://lists.apache.org/[email protected])).
 - Asking questions on [GitHub Discussions](https://github.com/apache/graphar/discussions/new?category=q-a).
 - Join our [Weekly Community Meeting](https://github.com/apache/incubator-graphar/wiki/GraphAr-Weekly-Community-Meeting).