Add doc for coverage option
Add doc for coverage option

calderas/ember-cli-blanket,sglanzer/ember-cli-blanket,dwickern/ember-cli-blanket,elwayman02/ember-cli-blanket,calderas/ember-cli-blanket,mike-north/ember-cli-blanket,yagni/ember-cli-blanket,yagni/ember-cli-blanket,elwayman02/ember-cli-blanket,notmessenger/ember-cli-blanket,notmessenger/ember-cli-blanket,dwickern/ember-cli-blanket,sglanzer/ember-cli-blanket,mike-north/ember-cli-blanket,jschilli/ember-cli-blanket,jschilli/ember-cli-blanket