Clarify relative references in on `img` tags

[cleverdevil]

Based upon behavior already in the wild (see http://feeds.kottke.org/json), clarify how relative image references on img tags should be interpreted, and encourage publishers to instead provide full URLs. This topic came up as a result of https://github.com/cleverdevil/together/issues/20

[manton]

Looks good, thanks!

[bcomnes]

This was always the most confusing part of atom/rss +1 to get this clarification in sooner than later.

[bcomnes]

Closes https://github.com/manton/JSONFeed/issues/8