Update install docs with #608 shiftfs issue resolution

[slfhstr]

Hit the shiftfs problem on kernel 5.15.48 Lost significant time trying & failing to solve it If it's a known problem, it should be referenced in the install guide https://github.com/nestybox/sysbox/blob/master/docs/user-guide/install-package.md. Going to try again but it would help lots of people if it was referenced with workaround in #608

[ctalledo]

Hi @timconsidine, thanks for trying Sysbox and sorry to hear you hit the issue with the Ubuntu 5.15.48 kernel and lost a bunch of time on it.

I was hoping that issue would be short-lived and expected Ubuntu to fix the kernel, but that has not been the case.

Thus I think your suggestion of updating the Sysbox docs to warn users is a good one. Will add it this week.

Thanks again and let us know if you got it to work. See issue 596 also for more info on the problem you hit.