Diamond: add starter implementation. (#1001)

* Diamond: add starter implementation.

* Add import statement

exercises/diamond/src/main/java/DiamondPrinter.java

@@ -0,0 +1,9 @@
+import java.util.List;
+
+class DiamondPrinter {
+
+    List<String> printToList(char a) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+}