Add f-droid badge (#8490)

# Thunderbird for Android

<a href="https://play.google.com/store/apps/details?id=net.thunderbird.android&referrer=utm_campaign%3Dandroid_metadata%26utm_medium%3Dweb%26utm_source%3Dgithub.com%26utm_content%3Dbadge" target="_blank"><img alt="Get it on Google Play" src="https://www.mozilla.org/media/img/l10n/en-US/firefox/android/btn-google-play-high-res.87d9720bbc8b.png" height="28"></a>

<a href="https://play.google.com/store/apps/details?id=net.thunderbird.android&referrer=utm_campaign%3Dandroid_metadata%26utm_medium%3Dweb%26utm_source%3Dgithub.com%26utm_content%3Dbadge" target="_blank"><img src="./docs/assets/get-it-on-play.png" alt="Get it on Google Play" height="28"></a>

<a href="https://f-droid.org/packages/net.thunderbird.android"><img src="./docs/assets/get-it-on-fdroid.png" alt="Get it on F-Droid" height="28"></a>

[![Latest release](https://img.shields.io/github/release/thunderbird/thunderbird-android.svg?style=for-the-badge&filter=THUNDERBIRD_*&logo=thunderbird)](https://github.com/thunderbird/thunderbird-android/releases/latest)

[![Latest beta release](https://img.shields.io/github/v/release/thunderbird/thunderbird-android.svg?include_prereleases&style=for-the-badge&label=beta&filter=THUNDERBIRD_*&logo=thunderbird)](https://github.com/thunderbird/thunderbird-android/releases)

Thunderbird for Android can be downloaded from a couple of sources:

- [Thunderbird on Google Play](https://play.google.com/store/apps/details?id=net.thunderbird.android&referrer=utm_campaign%3Dandroid_metadata%26utm_medium%3Dweb%26utm_source%3Dgithub.com%26utm_content%3Dlink)

- [Thunderbird Beta on Google Play](https://play.google.com/store/apps/details?id=net.thunderbird.android.beta&referrer=utm_campaign%3Dandroid_metadata%26utm_medium%3Dweb%26utm_source%3Dgithub.com%26utm_content%3Dlink)

- Thunderbird on [Google Play](https://play.google.com/store/apps/details?id=net.thunderbird.android&referrer=utm_campaign%3Dandroid_metadata%26utm_medium%3Dweb%26utm_source%3Dgithub.com%26utm_content%3Dlink) or [f-droid](https://f-droid.org/packages/net.thunderbird.android)

- Thunderbird Beta on [Google Play](https://play.google.com/store/apps/details?id=net.thunderbird.android.beta&referrer=utm_campaign%3Dandroid_metadata%26utm_medium%3Dweb%26utm_source%3Dgithub.com%26utm_content%3Dlink) or [f-droid](https://f-droid.org/packages/net.thunderbird.android.beta)

- [Github Releases](https://github.com/thunderbird/thunderbird-android/releases)

- F-Droid coming soon

- FFUpdater coming soon

By using Thunderbird for Android Beta, you have early access to current development and are able to try new features earlier.