Add DESCRIPTION file to apkindex

aports-turbo (the code that creates pkgs.alpinelinux.org) needs the DESCRIPTION inside the apkindex.tar.gz file to detect when the repository has changed and the update it's local sqlite database. the current implementation doesn't include it.

It can be added by adding a --description [date]-[hash] for example. the turbo implementations treats it as an opaque string and does description != olddescription to check if it's changed.

The DESCRIPTION in the alpine repository is v20191114-715-g0f1b0d0fb6 currently.