Add a notice about licensing to CONTRIBUTING.md

[waddlesplash]

This way we won't have to track everyone down again when we actually get around to relicensing :)

[waddlesplash]

cc @delroth @neobrain @lioncash

[comex]

LGTM if you change the date to a date in the future.

[waddlesplash]

@comex done

[Icekhaos]

Relicense? From what to what and why?

[pauldacheez]

@Icekhaos This has been discussed for a few months now. We're relicensing from GPLv2 to GPLv2+, which mainly just allows us to use libraries licensed under GPLv3 and other licenses that are compatible with v3 but not v2.

You may as well just get on the IRC (where we're most active) if you ever want to ask random questions – we're all on #dolphin-emu on Freenode.

[KAMiKAZOW]

I'd like to express my gratitude for this change. I hope you'll get as many past contributors to relicense as well. (My RPM spec file already has "any later version, so I'm good :) )

[Icekhaos]

@pauldacheez Some curious cats might find this useful, so an overview in tracker posts would be useful, or maybe that's just me. Anyway, that's awesome.