Add comments.

message.go

@@ -1,5 +1,7 @@
 package fcm
 
+// Message to send by Firebase Cloud Messaging Service.
+// https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages#resource:-message
 type Message struct {
 	// Union field target can be only one of the following:
 	Token     string `json:"token,omitempty"`
@@ -16,6 +18,7 @@ type Message struct {
 }
 
 // Basic notification template to use across all platforms.
+// https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages#notification
 type Notification struct {
 	Title string `json:"title,omitempty"`
 	Body  string `json:"body,omitempty"`