add guidance on archives (zipfiles / tarballs) of source code

[alee]

It's better to archive the plaintext source code than an archive of the source code (which is different from how things work on comses.net because we unpack the zipfiles automagically). There should be some guidance on that in this classroom.

The main points are plaintext source code can be diffed, versions managed etc., whereas a zipfile is just a raw binary blob that doesn't provide as much semantic details and the actual text differences between versions, etc., etc.