Add the metadata for org.lichess.mobileapp
Unfortunately, F-Droid is unable to build Cordova-based applications as of 20160831 (see https://gitlab.com/fdroid/fdroidserver/issues/60 ) so the metadata notes, that we have to distribute upstream apks (available on the GitHub project's page)
I know that redistributing the apks is an exception here, which is good. Though, I think that because it's not a problem with the software but a limitation of fdroidserver, we have no any other option.