Treat readium-sdk as a submodule

Just like the other Launchers for Readium, Readium-SDK should be a submodule. This is an easy change so I made it and this can either be merged or the commits can be cherry-picked into some other place.

As @bluefirepatrick pointed out in #1, we don't know who is in charge of the Windows Launcher so this is just an offer to help out where I know I can :smile: