dokuwiki: add warning about custom theme directory

roles/dokuwiki/README.md

@@ -26,6 +26,9 @@ None.
 - `dokuwiki__custom_theme_version`: Version (git branch, tag, ...) to checkout, default main
 - `dokuwiki__custom_theme_name`: Name of the directory into which the custom theme is cloned
 
+Warning: if a directory of the same name as `dokuwiki__custom_theme_name` already exists in the same directory, the task will fail.
+This needs to be manually deleted first before the first run with the the custom theme enabled.
+
 ## nginx Configuration
 
 This role does not configure your nginx server.