Update readme

Author: KennethWussmann

README.md

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-SimpleLogin is an e-mail aliasing service by Proton. This CLI tool allows to manage aliases via the terminal or in scripts.
+SimpleLogin is an e-mail aliasing service by Proton. This is a third-party CLI tool using [simplelogin-client](https://github.com/KennethWussmann/simplelogin-client) under the hood.
 
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 * [Installation](#installation)