Add F-Droid store badge

README.md

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 # Android Logcat without Root
 
-[![Get it on Google Play](http://www.tananaev.com/badges/google-play.svg)](https://play.google.com/store/apps/details?id=com.tananaev.logcat)
+[![Get it on Google Play](http://www.tananaev.com/badges/google-play.svg)](https://play.google.com/store/apps/details?id=com.tananaev.logcat) [![Get it on F-Droid](http://www.tananaev.com/badges/f-droid.svg)](https://f-droid.org/repository/browse/?fdid=com.tananaev.logcat)
 
 Read Android logs without root access. The app uses remote debugging to connect to phone local ADB daemon. Configuring remote debugging on a phone can be challenging and requires some technical expertise. It has to be done once and after each device reboot.