This is an openwrt feed that contains definitions to include CoreDNS software into your OpenWRT build.
see for details: https://openwrt.org/docs/guide-developer/feeds
open your feeds.conf.default and add the following line:
src-git feed_coredns https://code.tokarch.uk/mainnika/openwrt-feed-coredns.git
run update script:
./scripts/feeds update feed_coredns
install coredns packages into your build:
./scripts/feeds install -a -p feed_coredns