Authored by Bogdan Poplauschi

Added a draft for the Migration Guide document.

## 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];
```
... ...