Document backup suggestions

[ddpbsd]

Which files need to be backed up? That type of thing.

[nbuuck]

This may be satisfied by Migrating/backing up the manager.

[ddpbsd]

That always seemed incomplete, so I think it could be a good start. For instance, should /var/ossec/agentless/.passlist be included? What's the recommended process for restoring the server? I'm not sure what the best stopping point is, unfortunately.