hello-world: explain @Ignore annotations

exercises/hello-world/GETTING_STARTED.md

 $ gradle test
 ```
 
+To help you focus on one test at a time, [@Ignore](http://junit.sourceforge.net/javadoc/org/junit/Ignore.html)
+annotations have been added to every test but the first one. Any test with an `@Ignore` annotation will be skipped
+when you run the tests.
+
 ## Iterate through the tests
 
 After your first test passes, remove the `@Ignore` from the next test, and iterate on your solution,