Update README.md

[Poussinou]

Hi,

This small PR adds a button to get your app on F-Droid. You can later add a button for Google Play the same way : [<img src="https://play.google.com/intl/en_us/badges/images/generic/en-play-badge.png" alt="Get it on Google Play" height="80">]()

It also adds a link to get the latest APK from the Releases Section.

You can now eventually add the fdroid and/or f-droid tags that will appear on the right side of the repo.

close #24

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[!IMPORTANT] Add F-Droid and Google Play download buttons and APK link to README.md.

• Download Section:
  • Adds F-Droid download button with link to https://f-droid.org/packages/net.activitywatch.android/.
  • Adds Google Play download button with link to https://play.google.com/store/apps/details?id=net.activitywatch.android.
  • Adds link to download the latest APK from the Releases Section.
• Tags Suggestion:
  • Suggests adding fdroid and f-droid tags for repository visibility.

^{This description was created by for d6e0bad655124335a4f9af4cb3d4108cf7fe46a0. It will automatically update as commits are pushed.}