Add "How is SDWebImage better than X?" wiki link

README.md

@@ -16,6 +16,8 @@ It provides:
 - A guarantee that main thread will never be blocked
 - Performances!
 
+[How is SDWebImage better than X?](https://github.com/rs/SDWebImage/wiki/How-is-SDWebImage-better-than-X%3F)
+
 Who Use It
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