Add instructions to food-chain.
Add instructions to food-chain.

Since we're expecting people to write their own tests we need to remind them
that this is possible.

exercism/xruby,jdsayle/xruby,samilokan/xruby,Calvyn82/word_count,Insti/exercism-ruby,tommyschaefer/xruby,Insti/xruby,NeimadTL/ruby,Calvyn82/word_count,kickinbahk/xruby,gchan/xruby,darrencauthon/xruby,kotp/xruby,NobbZ/xruby,jdsayle/xruby,Insti/xruby,alexbrinkman/xruby,rashmiagar/exercism,samilokan/xruby,Calvyn82/xruby,wobh/xruby,wobh/xruby,Insti/exercism-ruby,remcopeereboom/xruby,exercism/xruby,Calvyn82/xruby,mamenama/xruby,NobbZ/xruby,tommyschaefer/xruby,rashmiagar/exercism,kickinbahk/xruby,remcopeereboom/xruby,alexbrinkman/xruby,mamenama/xruby,Insti/exercism-ruby,gchan/xruby,NeimadTL/ruby,darrencauthon/xruby,kotp/xruby,NeimadTL/ruby