Reasons for making this change:

There is no current gitignore option for the Crystal language.

Links to documentation supporting these rule changes:

The crystal init command generates a .gitignore file, among others. Out of the box, it looks like the file I've added in this PR. I've copied that .gitignore source into here.

The source code for how the crystal init file is created is here.

If this is a new template:

• Link to application or project’s homepage: https://crystal-lang.org