Add QR-Code to F-Droid page for adding repo more easily

fdroid.html

@@ -56,6 +56,8 @@
 
           <p>2) Open <a href="https://briarproject.org/fdroid/repo?fingerprint=1FB874BEE7276D28ECB2C9B06E8A122EC4BCB4008161436CE474C257CBF49BD6">this link</a> on your mobile device. (If you're using Firefox, long-click the link and choose <b>Open with F-Droid App</b>.)</p>
 
+          <p>Or scan this QR code on your phone:<br/><img src="img/fdroid/fdroid-add-repo-qr-code.png"/></p>
+
           <p>3) Choose <b>F-Droid</b>, then click <b>Just Once</b>.</p>
 
           <img src="img/fdroid/fdroid-app-chooser-cropped.png">