Update README.md

Author: egil

README.md

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-![AngleSharp Diffing](/docs/header.png)
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+  <img width="300" height="200" src="/docs/header.png">
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 # AngleSharp Diffing - A diff/compare library for AngleSharp
 [![Build status](https://ci.appveyor.com/api/projects/status/8awr3r4ylwy9habm?svg=true)](https://ci.appveyor.com/project/FlorianRappl/anglesharp-diffing)
 [![GitHub Tag](https://img.shields.io/github/tag/AngleSharp/AngleSharp.Diffing.svg?style=flat-square)](https://github.com/AngleSharp/AngleSharp.Diffing/releases)