Add screenshots

Author: cyanzhong

README.md

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 [MarkEdit](https://github.com/MarkEdit-app/MarkEdit) grammar checker based on [Harper](https://writewithharper.com/).
 
+<img src="./screenshots/01.png" width="480" title="Suggestion Card">
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+<img src="./screenshots/02.png" width="480" title="Review Problems">
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 ## Installation
 
 This extension is very big (24 MB) because it runs completely locally, install it by downloading `markedit-proofreading.js` from the [latest release](https://github.com/MarkEdit-app/MarkEdit-proofreading/releases/latest) and copying it to: