Added a draft for the Migration Guide document.

Docs/SDWebImage-4.0-Migration-guide.md

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+## SDWebImage 4.0 Migration Guide
+
+- To be created based on [CHANGELOG.md - 4.0.0 Backwards incompatible changes](https://github.com/rs/SDWebImage/blob/master/CHANGELOG.md#backwards-incompatible-changes)
+- See [Migration guide #1590](https://github.com/rs/SDWebImage/issues/1590)
+
+### New requirements: Xcode 8, iOS 7, Mac OS X 10.8, watchOS 2, tvOS 9
+- ...
+
+### Classes removed
+### Classes modified
+### Classes added
+
+##Migration
+
+### Simple case
+
+SDWebImage 3.x
+```
+[imageView sd_setImageWithURL:url];
+```
+
+SDWebImage 4.x
+```
+[imageView sd_setImageWithURL:url];
+```