Add fallback image URL docs

docs/configuration.md

@@ -186,6 +186,7 @@ You can set up a fallback image that will be used in case imgproxy can't fetch t
 * `IMGPROXY_FALLBACK_IMAGE_PATH`: path to the locally stored image;
 * `IMGPROXY_FALLBACK_IMAGE_URL`: fallback image URL.
 * `IMGPROXY_FALLBACK_IMAGE_HTTP_CODE`: HTTP code for the fallback image response. Default: `200`.
+* `IMGPROXY_FALLBACK_IMAGES_CACHE_SIZE`: <img class='pro-badge' src='assets/pro.svg' alt='pro' /> size of custom fallback images cache. When set to `0`, fallback images cache is disabled. By default 256 fallback images are cached.
 
 ## Skip processing
 

docs/generating_the_url.md

@@ -521,6 +521,19 @@ vts:%second
 
 Allows redefining `IMGPROXY_VIDEO_THUMBNAIL_SECOND` config.
 
+### Fallback image URL<img class='pro-badge' src='assets/pro.svg' alt='pro' />
+
+You can use a custom fallback image specifying its URL with `fallback_image_url` processing option:
+
+```
+fallback_image_url:%url
+fiu:%url
+```
+
+Where `url` is Base64-encoded URL of the custom fallback image.
+
+Default: blank
+
 ### Skip processing
 
 ```