Update the readme about the bitmap form of SVG

Author: dreampiggy

README.md

@@ -66,7 +66,7 @@ let package = Package(
 
 ## Usage
 
-### Use UIImageView
+### Render SVG as vector image
 
 To use SVG coder, you should firstly add the `SDImageSVGCoder` to the coders manager. Then you can call the View Category method to start load SVG images. See [Wiki - Coder Usage](https://github.com/SDWebImage/SDWebImage/wiki/Advanced-Usage#coder-usage) here for these steps.
 
@@ -106,10 +106,34 @@ imageView.frame = rect
 
 Note since UIImageView/NSImageView support this vector rendering, it means this coder plugin can be compatible for [SwiftUI](https://developer.apple.com/xcode/swiftui/). Check [SDWebImageSwiftUI](https://github.com/SDWebImage/SDWebImageSwiftUI/issues/50) for usage.
 
+## Render SVG as bitmap image
+
+In most cases, vector SVG is preferred. But however, sometimes you may want the bitmap form of SVG, used for image processing. You can use `.svgPrefersBitmap` to provide the bitmap format as well.
+
+By default it use the SVG canvas size. You can also specify a desired size during image loading using `.svgImageSize` context option. And you can specify whether or not to keep aspect ratio during scale using `.pdfImagePreserveAspectRatio` context option.
+
++ Objective-C
+
+```objectivec
+UIImageView *imageView;
+CGSize SVGImageSize = CGSizeMake(500, 500);
+[imageView sd_setImageWithURL:url placeholderImage:nil options:0 context:@{SDWebImageContextSVGImageSize : @(SVGImageSize)];
+```
+
++ Swift
+
+```swift
+let imageView: UIImageView
+let SVGImageSize = CGSize(width: 500, height: 500)
+imageView.sd_setImage(with: url, placeholderImage: nil, options: [], context: [.SVGImageSize : SVGImageSize])
+```
+
 ## Export SVG data
 
 `SDWebImageSVGCoder` provide an easy way to export the SVG image generated by this coder plugin, to the original SVG data.
 
+Note: The bitmap form of SVG does not support SVG data export.
+
 + Objective-C
 
 ```objectivec