.class files are ignored

Artifact files (like ```.class``` produced by ```javac```) should be ignored.
This PL contains a bare ```.gitignore``` file which instructs git to ignore .class files.

A more comprehensive ```.gitignore``` file for Java projects [can be found](https://www.gitignore.io/api/java) on [www.gitignore.io/](https://www.gitignore.io/). I personally prefer to add ignored files when the need arises.