Update README.md

[Poussinou]

Hi,

This small PR adds a link to get your app on F-Droid 👍

close #9

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

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[Poussinou]

I won't sign a CLA for such a contribution. You can cherry-pick my modification if you.