Add comments to author checks

entry.go

@@ -27,6 +27,10 @@ type Entry struct {
 	Extensions   []*ExtensionElement `xml:",any,omitempty"`
 }
 
+// atom:entry elements MUST contain one or more atom:author elements, unless
+// the atom:entry contains an atom:source element that contains an atom:author
+// element or, in an Atom Feed Document, the atom:feed element contains an
+// atom:author element itself.
 func (e *Entry) checkAuthors() error {
 	if e.Authors == nil {
 		if e.Source.Authors == nil {