Add LICENSE

[astrofrog]

As @bsipocz pointed out, we are missing a license... Any preference? We could just go with BSD?

[eteq]

Good point! Should we just take the one from https://github.com/dotastro/hack-list-submission-app/blob/master/LICENSE ? It has my name, though - should I change it to "the dotAstronomy team" or similar?