Added dev_mode to readme

Author: Richard Tabor

README.md

@@ -46,6 +46,7 @@ Other settings:
 * `merlin_url` — the admin url where Merlin WP will exist
 * `child_action_btn_url` — the url for the child theme generator's "Learn more" link
 * `help_mode` — a wizard for your wizard, if you need help *(beta)*
+* `dev_mode` — retain the "Theme Setup" menu item under the WordPress Admin > Appearance section for testing. Also enables JS/CSS minified files.
 * `branding` — show Merlin WP's logo or not *(beta)*
 
 ### 3. Add your demo content