[Tagging] sport=rugby

[John Smith]

Currently the neither the sport wiki page nor the sport=rugby wiki
page define what sport=rugby actually is, and I can't find anything
for 'union' or 'league' so I don't think this has been documented
properly.

While I assume the current tag is for rugby union, and one solution to
this might be to sub-tag sport=rugby, rugby=union/league but this
seems a bit hackish to me and I think we should do things properly and
have sport=rugby_union/rugby_league

Anyone else think of a suitable solution for this?