docs: update README with resend from instruction (#5)

* update README with resend from instruction

* docs: improve wording of onboarding@resend.dev instructions

---------

Co-authored-by: ernesto <ernesto@pigmalion.com.ar>
Co-authored-by: bashbunni <bunni@bashbunni.dev>

README.md

@@ -72,6 +72,9 @@ environment to pre-fill the field anytime you launch `pop`.
 export POP_FROM=pop@charm.sh
 export POP_SIGNATURE="Sent with [Pop](https://github.com/charmbracelet/pop)!"
 ```
+> **Note**:
+> If you wish to use a resend account without a custom domain, you can use
+> `onboarding@resend.dev` to send emails.
 
 ## Installation