Mention a method to iterate over all files

[nicorikken]

The common use-case for people starting with REUSE is to apply the headers to all files in the repository. The tutorial website doesn't currently hint at such a method, resulting in people wondering how to do so (I've had this question twice after pointing to the official docs). As was mentioned in a REUSE github issue already, this can be done by using $ git ls-files | xargs reuse addheader .... Hinting at the piping options would be helpful to people wanting to adopt REUSE.

I propose to mention it in these locations:

• Tutorial in the 2. Add copyright and licensing information to each file section
• Addheader documentation (I know, different repository)

What do you think?

[mxmehl]

Definitely, but it would even be better if we had fsfe/reuse-tool#158 done before that.

fsfe/reuse-tool#291 also seems a bit connected to your issue.