Require symbols for JP ROMs for new Pull Requests about pmdsky-debug

[UsernameFodder]

This seems kind of counter to a core design goal of the repo and resymgen, which is to encourage "just add what you have" contributions. When most people do research, they're usually only looking at one version at a time. There's already plenty of symbols in the repo that are NA-only or EU-only.

I think it's better to have incomplete information than no information at all. If there were a requirement that contributions must be multi-version, it would be pretty stifling. If there are symbols we really care about that aren't addressed in all versions, people are welcome to make pull requests that add them, and having the symbols in other version even helps with that by enabling cross-referencing.

We could do nothing, but this would probably eventually lead to issues of the JP ROMs being barely supported by projects using pmdsky-debug.

IMO this isn't really in the scope of pmdsky-debug. This repo just provides all the information that's known. Verifying whether or not the JP offsets are present is a matter that ought to be up to users of the symbol tables.

A document describing contribution should be added (unless there is already one and I'm just blind)

There are already is one.