Update the readme and screenshot about detailed usage

Author: dreampiggy

Example/Screenshot/SVGDemo.png

@@ -5,32 +5,48 @@
 [![License](https://img.shields.io/cocoapods/l/SDWebImageSVGCoder.svg?style=flat)](https://cocoapods.org/pods/SDWebImageSVGCoder)
 [![Platform](https://img.shields.io/cocoapods/p/SDWebImageSVGCoder.svg?style=flat)](https://cocoapods.org/pods/SDWebImageSVGCoder)
 
+## What's for
+SDWebImageSVGCoder is a SVG coder plugin for [SDWebImage](https://github.com/rs/SDWebImage/) framework, which provide the image loading support for [SVG](https://en.wikipedia.org/wiki/Scalable_Vector_Graphics) using [SVGKit](https://github.com/SVGKit/SVGKit) SVG engine.
+
 ## Example
 
 To run the example project, clone the repo, and run `pod install` from the Example directory first.
 
+You can modify the code or use some other SVG files to check the compatibility.
+
 ## Requirements
 
++ iOS 8
+
 ## Installation
 
 SDWebImageSVGCoder is available through [CocoaPods](https://cocoapods.org). To install
 it, simply add the following line to your Podfile:
 
 ```ruby
 pod 'SDWebImageSVGCoder'
+pod 'SVGKit', :git => 'https://github.com/SVGKit/SVGKit.git', :branch => '2.x'
 ```
 
+**Note:** Because the dependency third party library [SVGKit](https://github.com/SVGKit/SVGKit#versions) does not release the 2.x version, and the current latest release version is not compatible for Xcode 9/10. So you should specify `2.x` branch dependency or using commit-id to import the dependency through CocoaPods.
+
 ## Usage
 
+### Use UIImageView (render SVG as bitmap 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.
 
+Because SVG is a [vector image](https://en.wikipedia.org/wiki/Vector_graphics) format, which means it does not have a fixed bitmap size. However, `UIImage` or `CGImage` are all [bitmap image](https://en.wikipedia.org/wiki/Raster_graphics). For `UIImageView`, we will only parse SVG with a fixed image size (from the SVG viewPort information). But we also support you to specify a desired size during image loading using `vectorImageSize` context option.
+
 + Objective-C
 
-```objective-c
+```objectivec
 SDImageSVGCoder *SVGCoder = [SDImageSVGCoder sharedCoder];
 [[SDImageCodersManager sharedManager] addCoder:SVGCoder];
 UIImageView *imageView;
-[imageView sd_setImageWithURL:url];
+// this arg is optional, if don't provide, use the viewport size instead
+CGSize vectorImageSize = CGSizeMake(100, 100);
+[imageView sd_setImageWithURL:url placeholderImage:nil options:0 context:@{SDWebImageContextVectorImageSize : @(vectorImageSize)];
 ```
 
 + Swift
@@ -40,6 +56,37 @@ let SVGCoder = SDImageSVGCoder.shared
 SDImageCodersManager.shared.addCoder(SVGCoder)
 let imageView: UIImageView
 imageView.sd_setImage(with: url)
+// this arg is optional, if don't provide, use the viewport size instead
+let vectorImageSize = CGSize(width: 100, height: 100)
+imageView.sd_setImage(with: url, placeholderImage: nil, options: [], context: [.vectorImageSize : vectorImageSize])
+```
+
+### Use SVGKImageView (render SVG as vector image)
+
+[SVGKit](https://github.com/SVGKit/SVGKit) also provide some built-in image view class for vector image loading (scale to any size without losing detail). The `SVGKLayeredImageView` && `SVGKFastImageView` are the subclass of `SVGKImageView` base class. We supports these image view class as well. You can just use the same API like normal `UIImageView`.
+
+For the documentation about `SVGKLayeredImageView`, `SVGKFastImageView` or `SVGKImageView`, check [SVGKit](https://github.com/SVGKit/SVGKit) repo for more information.
+
+**Note**: If you only use these image view class and don't use SVG on `UIImageView`, you don't need to register the SVG coder to coders manager. These image view loading was using the [Custom Image Class](https://github.com/rs/SDWebImage/wiki/Advanced-Usage#customization) feature of SDWebImage.
+
+**Attention**: These built-in image view class does not works well on `UIView.contentMode` property, you need to re-scale the layer tree after image was loaded. We provide a simple out-of-box solution to support it. Set the `sd_adjustContentMode` property to `YES` then all things done.
+
++ Objective-C
+
+```objectivec
+SVGKImageView *imageView; // can be either `SVGKLayeredImageView` or `SVGKFastImageView`
+imageView.contentMode = UIViewContentModeScaleAspectFill;
+imageView.sd_adjustContentMode = YES; // make `contentMode` works
+[imageView sd_setImageWithURL:url];
+```
+
++ Swift:
+
+```swift
+let imageView: SVGKImageView // can be either `SVGKLayeredImageView` or `SVGKFastImageView`
+imageView.contentMode = .aspectFill
+imageView.sd_adjustContentMode = true // make `contentMode` works
+imageView.sd_setImage(with: url)
 ```
 
 ## Screenshot

README.md

@@ -2,7 +2,7 @@
 //  SDImageSVGCoder.h
 //  SDWebImageSVGCoder
 //
-//  Created by lizhuoli on 2018/9/27.
+//  Created by DreamPiggy on 2018/9/27.
 //
 
 #import <SDWebImage/SDWebImage.h>

SDWebImageSVGCoder/Classes/SDImageSVGCoder.h

@@ -2,7 +2,7 @@
 //  SDImageSVGCoder.m
 //  SDWebImageSVGCoder
 //
-//  Created by lizhuoli on 2018/9/27.
+//  Created by DreamPiggy on 2018/9/27.
 //
 
 #import "SDImageSVGCoder.h"

SDWebImageSVGCoder/Classes/SDImageSVGCoder.m

@@ -2,7 +2,7 @@
 //  SDSVGImage.h
 //  SDWebImageSVGCoder
 //
-//  Created by lizhuoli on 2018/10/10.
+//  Created by DreamPiggy on 2018/10/10.
 //
 
 #import <SDWebImage/SDWebImage.h>

SDWebImageSVGCoder/Classes/SDSVGImage.h

@@ -2,7 +2,7 @@
 //  SDSVGImage.m
 //  SDWebImageSVGCoder
 //
-//  Created by lizhuoli on 2018/10/10.
+//  Created by DreamPiggy on 2018/10/10.
 //
 
 #import "SDSVGImage.h"

SDWebImageSVGCoder/Classes/SDSVGImage.m

@@ -2,7 +2,7 @@
 //  SDWebImageSVGCoderDefine.h
 //  SDWebImageSVGCoder
 //
-//  Created by lizhuoli on 2018/10/11.
+//  Created by DreamPiggy on 2018/10/11.
 //
 
 #import <SDWebImage/SDWebImage.h>

SDWebImageSVGCoder/Classes/SDWebImageSVGCoderDefine.h

@@ -2,7 +2,7 @@
 //  SDWebImageSVGCoderDefine.m
 //  SDWebImageSVGCoder
 //
-//  Created by lizhuoli on 2018/10/11.
+//  Created by DreamPiggy on 2018/10/11.
 //
 
 #import "SDWebImageSVGCoderDefine.h"

SDWebImageSVGCoder/Classes/SDWebImageSVGCoderDefine.m

@@ -2,7 +2,7 @@
 //  SVGKImageView+WebCache.h
 //  SDWebImageSVGCoder
 //
-//  Created by lizhuoli on 2018/10/10.
+//  Created by DreamPiggy on 2018/10/10.
 //
 
 #import <SVGKit/SVGKit.h>

SDWebImageSVGCoder/Classes/SVGKImageView+WebCache.h

@@ -2,7 +2,7 @@
 //  SVGKImageView+WebCache.m
 //  SDWebImageSVGCoder
 //
-//  Created by lizhuoli on 2018/10/10.
+//  Created by DreamPiggy on 2018/10/10.
 //
 
 #import "SVGKImageView+WebCache.h"