Update the demo for placeholder comments

Author: dreampiggy

Example/SDWebImageSwiftUIDemo/ContentView.swift

@@ -86,6 +86,9 @@ struct ContentView: View {
                             #if os(macOS) || os(iOS) || os(tvOS)
                             AnimatedImage(url: URL(string:url))
                             .indicator(SDWebImageActivityIndicator.medium)
+                            /**
+                            .placeholder(UIImage(systemName: "photo"))
+                            */
                             .transition(.fade)
                             .resizable()
                             .scaledToFit()
@@ -100,6 +103,11 @@ struct ContentView: View {
                             #if os(macOS) || os(iOS) || os(tvOS)
                             WebImage(url: URL(string:url))
                             .resizable()
+                            /**
+                             .placeholder {
+                                 Image(systemName: "photo")
+                             }
+                             */
                             .indicator(.activity)
                             .animation(.easeInOut(duration: 0.5))
                             .transition(.fade)

README.md

@@ -90,8 +90,11 @@ var body: some View {
             // Success
         }
         .resizable() // Resizable like SwiftUI.Image
+        .placeholder {
+            Image(systemName: "photo") // Placeholder
+        }
         .indicator(.activity) // Activity Indicator
-        .animation(.easeInOut(duration: 0.5))
+        .animation(.easeInOut(duration: 0.5)) // Animation Duration
         .transition(.fade) // Fade Transition
         .scaledToFit()
         .frame(width: 300, height: 300, alignment: .center)
@@ -118,6 +121,7 @@ var body: some View {
             // Error
         }
         .resizable() // Actually this is not needed unlike SwiftUI.Image
+        .placeholder(UIImage(systemName: "photo")) // Placeholder
         .indicator(SDWebImageActivityIndicator.medium) // Activity Indicator
         .transition(.fade) // Fade Transition
         .scaledToFit() // Attention to call it on AnimatedImage, but not `some View` after View Modifier