Update the readme about detail coders usage, accept header modification, etc

Author: dreampiggy

README.md

@@ -37,40 +37,100 @@ github "SDWebImage/SDWebImageWebPCoder"
 
 ## Usage
 
+### Add Coder
+
+Before using SDWebImage to load WebP images, you need to register the WebP Coder to your coders manager. This step is recommended to be done after your App launch (like AppDelegate method).
+
 + Objective-C
 
 ```objective-c
 // Add coder
 SDImageWebPCoder *webPCoder = [SDImageWebPCoder sharedCoder];
 [[SDImageCodersManager sharedManager] addCoder:webPCoder];
+```
+
++ Swift
+
+```swift
+// Add coder
+let WebPCoder = SDImageWebPCoder.shared
+SDImageCodersManager.shared.addCoder(WebPCoder)
+```
+
+### Modify HTTP Accept Header
+
+Some of image server provider may try to detect the client supported format, by default, SDWebImage use `image/*,*/*;q=0.8` for [Accept](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept). You can modify it with the `image/webp` as well.
+
++ Objective-C
 
+```objective-c
+[[SDWebImageDownloader sharedDownloader] setValue:@"image/webp,image/*,*/*;q=0.8" forHTTPHeaderField:@"Accept"];
+```
+
++ Swift
+
+```swift
+SDWebImageDownloader.shared.setValue("image/webp,image/*,*/*;q=0.8", forHTTPHeaderField:"Accept")
+```
+
+### Loading
+
++ Objective-C
+
+```objective-c
 // WebP image loading
 UIImageView *imageView;
 NSURL *webpURL;
 [imageView sd_setImageWithURL:webpURL];
-
-// WebP image encoding
-UIImage *image;
-NSData *webpData = [[SDImageWebPCoder sharedCoder] encodedDataWithImage:image format:SDImageFormatWebP options:nil];
 ```
 
 + Swift
 
 ```swift
-// Add coder
-let WebPCoder = SDImageWebPCoder.shared
-SDImageCodersManager.shared.addCoder(WebPCoder)
-
 // WebP online image loading
 let webpURL: URL
 let imageView: UIImageView
 imageView.sd_setImage(with: webpURL)
+```
 
+### Decoding
+
++ Objective-C
+
+```objective-c
+// WebP image decoding
+NSData *webpData;
+UIImage *image = [[SDImageWebPCoder sharedCoder] decodedImageWithData:webpData options:nil];
+```
+
++ Swift
+
+```swift
+// WebP image decoding
+let webpData: Data
+let image = SDImageWebPCoder.shared.decodedImage(with: data, options: nil)
+```
+
+### Encoding
+
++ Objective-c
+
+```objective-c
+// WebP image encoding
+UIImage *image;
+NSData *webpData = [[SDImageWebPCoder sharedCoder] encodedDataWithImage:image format:SDImageFormatWebP options:nil];
+```
+
++ Swift
+
+```swift
 // WebP image encoding
 let image: UIImage
 let webpData = SDImageWebPCoder.shared.encodedData(with: image, format: .webP, options: nil)
 ```
 
+See more documentation in [SDWebImage Wiki - Coders](https://github.com/SDWebImage/SDWebImage/wiki/Advanced-Usage#custom-coder-420)
+
 ## Example
 
 To run the example project, clone the repo, and run `pod install` from the Example directory first.