Add Java learning resources

docs/LEARNING.md

+## Recommended learning resources
+
+* [The Java Tutorials](https://docs.oracle.com/javase/tutorial/index.html)
+* [Java SE API Documentation](https://docs.oracle.com/javase/8/docs/api/index.html)
+* [Learn X in Y minutes](https://learnxinyminutes.com/docs/java/)
+* [Free Programming Books - Java](https://github.com/vhf/free-programming-books/blob/master/free-programming-books.md#java)
+* [Intro to Java Programming](https://www.udacity.com/course/intro-to-java-programming--cs046)
+* [Java Tutorial for Complete Beginners](https://www.udemy.com/java-tutorial/)
+* [Stack Overflow](https://stackoverflow.com/questions/tagged/java)