Example of "reasonable distribution cost" added

As per discussion in the mailing list here: http://lists.okfn.org/pipermail/open-data-handbook/2013-March/000307.html
Merge pull request #10 from peterk/patch-1

Example of "reasonable distribution cost" added.

(I'm not super thrilled with 'DVD' which will seem dated soon, but I guess a concrete example is easier to understand than something like "storage media".)