Update README.md

Author: xpmn

README.md

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+[<img src="https://blog.mozilla.org/addons/files/2020/04/get-the-addon-fx-apr-2020.svg" alt="for Firefox" height="60px">](https://addons.mozilla.org/en-US/firefox/addon/to-deepl/)
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 # To DeepL
 
 Firefox extension. Highlight text on a web page and send it to DeepL.