The "User" package is practically said to not being touched by anything (except for settings modifications from within plugins). That is, it's considered to be your "test stuff here" or your "small things/settings/snippets and stuff that does not really belong anywhere else". Thus, it's easy to create a subdir in the User package and store a few settings or snippets there. If you intend to create more than just a few files it might be worth creating a new package for them. This is mostly the case for new languages with e.g. snippets.
Well, there are no real "rules" for it but this is how I organize my things. And, if you want to write a plugin/package and upload it to github/Package Control you really want this to be in its own directory.