readme: Mention direct APK downloads, for both release and beta.
readme: Mention direct APK downloads, for both release and beta.

We've been consistently uploading our APK builds to GitHub, as
part of our release process, since last year.  Make sure people
who come looking for them don't get told they're only available
through the Play Store.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile