Create RESOURCES.md and add resources

docs/RESOURCES.md

+## Useful Resources
+
+* [StackOverflow](http://stackoverflow.com/questions/tagged/java).
+* [The official java subreddit](https://www.reddit.com/r/java)
+* [The exercism/java Gitter room](https://gitter.im/exercism/java)
+* [Java documentation](https://docs.oracle.com/javase/8/)
+* [Java Programming Books](https://github.com/vhf/free-programming-books/blob/master/free-programming-books.md#java)