clarify transactional example (#148)

README.md

@@ -303,7 +303,7 @@ Create a new `SendEmailRequest` object containing:
 
 - `transactional_message_id`: the ID of the transactional message you want to send, or the `body`, `from`, and `subject` of a new message.
 - `to`: the email address of your recipients
-- an `identifiers` object containing the `id` of your recipient. If the `id` does not exist, Customer.io will create it.
+- an `identifiers` object containing the email and/or `id` of your recipient. If the person you reference by email or ID does not exist, Customer.io creates them.
 - a `message_data` object containing properties that you want reference in your message using Liquid.
 - You can also send attachments with your message. Use `attach` to encode attachments.
 
@@ -326,7 +326,7 @@ const request = new SendEmailRequest({
     products: [],
   },
   identifiers: {
-    id: "2",
+    email: "person@example.com",
   },
 });