Update README: added f-droid link

Author: nkanaev

README.md

@@ -13,6 +13,10 @@ Android comic book reader.
 * Simple file browser
 * Free, open-source, no ads, gluten-free
 
+<a href="https://f-droid.org/repository/browse/?fdid=com.nkanaev.comics">
+  <img height="44" width="auto" src='https://upload.wikimedia.org/wikipedia/commons/0/0d/Get_it_on_F-Droid.svg'></img>
+</a>
+
 ## License
 
 Source code is available under the MIT license. See the LICENSE for more info.