Readme: Add F-Droid link

Readme.md

@@ -2,7 +2,11 @@
 
 [![YouTube](fastlane/metadata/android/en-US/images/featureGraphic.png)](https://www.youtube.com/channel/UCbGKXwhh1DkuINyZw05kyHw/featured)
 
-[![Google Play](https://play.google.com/intl/en_us/badges/images/generic/en-play-badge.png)](https://play.google.com/store/apps/details?id=com.calcitem.sanmill)
+<a href="https://f-droid.org/packages/com.calcitem.sanmill/" target="_blank">
+<img src="https://f-droid.org/badge/get-it-on.png" alt="Get it on F-Droid" height="80"/></a>
+
+<a href="https://play.google.com/store/apps/details?id=com.calcitem.sanmill" target="_blank">
+<img src="https://play.google.com/intl/en_us/badges/images/generic/en-play-badge.png" alt="Get it on Google Play" height="80"/></a>
 
 [![Build Status](https://ci.appveyor.com/api/projects/status/github/Calcitem/Sanmill?branch=master&svg=true)](https://ci.appveyor.com/project/Calcitem/Sanmill/branch/master)
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/a09e5b5ab4f04b1585acb478af0bd593)](https://app.codacy.com/manual/calcitem/Sanmill?utm_source=github.com&utm_medium=referral&utm_content=calcitem/Sanmill&utm_campaign=Badge_Grade_Settings)
@@ -88,4 +92,4 @@ to where the source code can be found. If you make any changes to the
 source code, these changes must also be made available under the GPL.
 
 For full details, read the copy of the GPL v3 found in the file named
-*Copying.txt*.
+*Copying.txt*.