README: Add note on Transient 0.7.4 requirement for gptel

* README.org (Installation): Mention explicitly that Transient
0.7.4 or higher is required.  This is a frequent issue because of
Emacs' reluctance to update built-in packages (#776).

This instruction should apply to Elpaca/Straight too, but I don't
know how to ensure that Transient is updated when using these
package managers.  (A PR is welcome.)

Author: karthink

README.org

@@ -169,6 +169,8 @@ gptel uses Curl if available, but falls back to the built-in url-retrieve to wor
 
 ** Installation
 
+Note: gptel requires Transient 0.7.4 or higher.  Transient is a built-in package and Emacs does not update it by default.  Ensure that =package-install-upgrade-built-in= is true, or update Transient manually.
+
 - *Release version*: =M-x package-install= ⏎ =gptel= in Emacs.
 - *Development snapshot*: Add MELPA or NonGNU-devel ELPA to your list of package sources, then install with =M-x package-install= ⏎ =gptel=.
 - *Optional:* Install =markdown-mode=.
@@ -183,6 +185,8 @@ gptel uses Curl if available, but falls back to the built-in url-retrieve to wor
 #+html: <details><summary>
 **** Manual
 #+html: </summary>
+Note: gptel requires Transient 0.7.4 or higher.  Transient is a built-in package and Emacs does not update it by default.  Ensure that =package-install-upgrade-built-in= is true, or update Transient manually.
+
 Clone or download this repository and run =M-x package-install-file⏎= on the repository directory.
 #+html: </details>
 #+html: <details><summary>