Update README with Play Store beta testing track, Amazon link, F-Droid info (#397)

Update README with Play Store beta testing track, Amazon link, F-Droid clarifications

Signed-off-by: Andrea Gottardo <andrea@tailscale.com>

README.md

@@ -10,13 +10,20 @@ This repository contains the open source Tailscale Android client.
 
 ## Using
 
-[<img src="https://fdroid.gitlab.io/artwork/badge/get-it-on.png"
-     alt="Get it on F-Droid"
-     height="80">](https://f-droid.org/packages/com.tailscale.ipn/)
 [<img src="https://play.google.com/intl/en_us/badges/images/generic/en-play-badge.png"
      alt="Get it on Google Play"
      height="80">](https://play.google.com/store/apps/details?id=com.tailscale.ipn)
 
+Help us test new features and bug-fixes before they ship to all users! A [beta testing track](https://play.google.com/apps/testing/com.tailscale.ipn) is available on the Play Store. 
+
+#### Amazon Appstore
+
+The app can be downloaded from the [Amazon Appstore](https://www.amazon.com/dp/B0D38TRB3N) for Amazon Fire tablets and Fire TV devices.
+
+#### F-Droid
+
+The [F-Droid](https://f-droid.org/packages/com.tailscale.ipn/) project builds the source code in this repository and maintains independently-built APKs. Note that F-Droid builds are not released, updated, or verified by the Tailscale team.
+
 ## Preparing a build environment
 
 There are several options for setting up a build environment. The Android Studio