Document exact specifics of udev dependancy

[josephlr]

With #54, the directory /dev/disk/by-uuid and its contents are now necessary to establish cross filesystem links. This means use of these links now depends on a specific default udev rule, namely 60-persistent-storage.rules. The source can be found in the systemd tree, eudev tree, and the old udev tree.

The documentation (probably part of #10) should explain this an other dependencies in a single section. It should also explain when these dependencies are necessary (build-time, run-time, link creation time, etc..).