add Troubleshooting guide

[miabbott]

migrated from https://pagure.io/fedora-docs/silverblue/issue/20

Since we still have some rough edges to smooth out, it would be beneficial to users to have a Troubleshooting guide that covers some of the more common errors we've seen.

First couple of ideas:

• /boot/efi read-only on Mac systems (or HFS+ filesystems) - Silverblue regression: EFI+Mac: bootloader write error coreos/rpm-ostree#1380
• Forbidden base package replacements - split base/layered versionlocked package problem (krb5-workstation) coreos/rpm-ostree#415
• Can't install RPMs that write to /optetc - Determine support path for RPMs that install into /opt, /usr/local etc coreos/rpm-ostree#233
• Can't boot after upgrade; SELinux policy recompilation - Handle SELinux policy recompilation ostreedev/ostree#1026
• Can't install nVidia drivers; DKMS support - Implement compatibility with DKMS (Nvidia, etc.) coreos/rpm-ostree#1091
• can't use usermod -aG group <user> - Drop requirement on nss-altfiles, use systemd sysusers coreos/rpm-ostree#49
  • Workaround: copying the group entry from /usr/lib/group to /etc/group
• ostree fsck reports corrupted file
  • Workaround: delete corrupted object, mark commitpartial, re-pull - POC: fsck repair ostreedev/ostree#345 (comment)
• Reparing bad SELinux policy - https://discussion.fedoraproject.org/t/lots-of-avc-denied-systemd-journald-in-journalctl-log/1601/3