java/rotational-cipher: create mentor notes

Create some mentor notes for the java/rotational-cipher exercise.

Most importantly it should include one or more reasonable solutions (at least the general structure of a reasonable solution).

Locate the mentor-notes conventions for the java/rotational-cipher exercise in the website-copy repository and review comparable exercise notes first. Add mentor guidance that includes at least one reasonable solution or its general structure; done means the notes are present and cover the requested solution guidance.

Written by the indexing model from the issue text.