Add note about minitest 4 vs 5 in README
Add note about minitest 4 vs 5 in README

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