Update PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,9 +15,11 @@ Please include the name of the theme.
 
 Have your run the python script? `python3 ./scripts/gen_theme_previews.py <folder-name-eg-standard>`
 
-## Notes:
+## Notes
 
 We cannot accept pull request that include custom background images because:
 
-- licensing
-- we are trying to keep the binary size of the repo as small as possible (just the yaml files)
+* of licensing restrictions
+* we are trying to keep the binary size of the repo as small as possible (just the yaml files)
+
+If your theme has an intended custom background image, include a comment in the yaml with a link to where people should download it.