[elrepo] Mention -devel packages for kernels

[Joseph Christopher Sible]

When I initially installed the kernel-ml package on my system, I got the following error:

Error! echo
Your kernel headers for kernel 4.18.5-1.el7.elrepo.x86_64 cannot be found at
/lib/modules/4.18.5-1.el7.elrepo.x86_64/build or /lib/modules/4.18.5-1.el7.elrepo.x86_64/source.

Installation still reported that it was successful, though, and the kernel booted fine. What
didn't work fine were my out-of-tree modules that DKMS is supposed to build. I discovered that
the package I needed to fix this was kernel-ml-devel. Can a note mentioning the -devel packages
be added to the https://elrepo.org/tiki/kernel-lt and https://elrepo.org/tiki/kernel-ml pages,
to warn other users installing these kernels of this problem?

Thanks,

Joseph C. Sible