Fix #109 -- Add USE_PLACEHOLDERS setting documentation (#113)

Author: Nekidev

README.md

@@ -85,6 +85,7 @@ PICTURES = {
     "CONTAINER_WIDTH": 1200,
     "FILE_TYPES": ["WEBP"],
     "PIXEL_DENSITIES": [1, 2],
+    "USE_PLACEHOLDERS": True
 }
 ```