pig-latin: add hints about no starter implementation

[FridaTveit]

According to our starter implementation policy all exercises with difficulty 4 or lower should have starter implementation. It can be a bit confusing for users when they do their first exercise without starter implementation. Therefore we've decided to add a hints.md file to all exercises with difficulty 5 (see the policy doc for more details and issue #1084 for a discussion about why we decided to do this).

Please add this hints.md file to the pig-latin exercise. The file can be copied from the flatten-array exercise.

Once a hints.md file has been added to an exericse, that exercise's readme needs to be regenerated. This is because the hints.md file will get appended to the readme when the readme is generated. To generate the readme you need to:

1. Download configlet and put it somewhere on your path (configlet is the tool we use to generate reame files).
2. Clone the problem-specifications repository (this repo contains the template we use to generate readmes).
3. Run configlet generate . --only pig-latin --spec-path path_to_problem_specifications from the root of this repository (exercism/java).

Once that's done you can commit and push the changes to the readme and the new hints.md file :)