Updated README to include issue reporting (#12)

Author: asadullahnaweed

README.md

@@ -10,3 +10,7 @@ Please note, we do not accept pull requests.
 
 * [Quickstart for Android Java](//developers.google.com/ar/develop/java/quickstart)
 * [ARCore SDK for Java API Reference](//developers.google.com/ar/reference/java)
+
+## Reporting Issues
+
+Any issues with this codelab should be reported in the main [ARCore Issue Tracker](https://github.com/google-ar/arcore-android-sdk/issues).