Add note to add support for sentAt.

It's in the spec, and I get it from the server, but it's not in the
client library I'm using.

src/client/types.tsx

@@ -11,7 +11,9 @@ export interface IMailbox extends client.IMailboxProperties {
 	emailIds: Array<string> | null;
 }
 
-export interface IEmail extends client.IEmailProperties {}
+export interface IEmail extends client.IEmailProperties {
+	//sentAt: IutcDate|null,
+}
 
 export interface IAccount extends client.IAccount {
 	id: string;