PKGBUILD installs to /usr/share/docs not /usr/share/doc.
Filesystem Hierarchy Standard - 4.11. /usr/share : Architecture-independent data
Current:
install -Dm644 README.md -t "$pkgdir/usr/share/docs/$pkgname"
Should be:
install -Dm644 README.md -t "$pkgdir/usr/share/doc/$pkgname"