docs: update

README.md

@@ -111,25 +111,33 @@ $scale: 0.5; // the scale of loading image in place holder
 - root
  The element that is used as the viewport for checking visiblity of the target. Must be the ancestor of the target. Defaults to the browser viewport if not specified or if null.
 
-- rootMargin 
+- rootMargin
+
  Margin around the root. Can have values similar to the CSS margin property, e.g. "10px 20px 30px 40px" (top, right, bottom, left). The values can be percentages. This set of values serves to grow or shrink each side of the root element's bounding box before computing intersections. Defaults to all zeros.
 
 - threshold
+
  Either a single number or an array of numbers which indicate at what percentage of the target's visibility the observer's callback should be executed. If you only want to detect when visibility passes the 50% mark, you can use a value of 0.5. If you want the callback run every time visibility passes another 25%, you would specify the array [0, 0.25, 0.5, 0.75, 1]. The default is 0 (meaning as soon as even one pixel is visible, the callback will be run). A value of 1.0 means that the threshold isn't considered passed until every pixel is visible.
 
 - blurFilter
+
  If you just need blur filter for loading image. Set a number to it.
 
 - filter
+
  If you want to set more [filter](https://developer.mozilla.org/en-US/docs/Web/CSS/filter). It uses the input string as css filter.
 
 - placeHolderImageSrc
+
  src for loading image. It works just like src, takes base64 or url.
 
+- imageRatio
+
+ image width / height
+
 For `ngx-image-placeholder` directive, it takes
 
 - imageRatio
- image width / height
 
 - placeHolderImageSrc