Add crystal linux to setup page
By the way, as I mentioned under another PR, I probably won't be merging this PR at the moment. There is already too many distros in the list (see https://github.com/flatpak/flatpak.github.io/issues/471) and I am not sure how popular is Crystal Linux (it looks to be a new distro). Anyway, feel free to keep the PR open, I may merge it in (near) future. Thanks!
@axtloss This is probably fine to be merged now after the merge conflict and the duplicate entry issue are resolved.
Applied your suggestion and the merge conflict should be resolved too now
Looks good now, thanks! :-)