register the overlay with Gentoo/Layman

[pavlix]

The README already says you can use layman -a kernel but this isn't true as the overlay hasn't yet been registered. Let's do it.

[pavlix]

Until then you can use the following /etc/portage/repos.conf/kernel.conf.

[kernel]
location = /usr/local/overlay/kernel
sync-type = git
sync-uri = https://github.com/crossdistro/gentoo-kernel
auto-sync = yes