Update README.md

updated readme to include an implied aspect

Author: Lurking987

plugins/tmdb-backdrop/README.md

@@ -2,7 +2,8 @@
 This plugin looks to see if the Group has a TMDB URL.
 Grabs backdrop from TMDB and places it as a background image. 
 ### **_Required for this plugin to work_**:
-1. Your own TMDB API. 
+1. Your own TMDB API.
+2. The movie's TMDB URL added as one of the Group's URL. (If adding the TMDB URL for the first time, refresh the page to see the effect)
 
 ### Steps Needed:
 - Add your TMDB API into the plugin's settings