README: Point to pkg.go.dev instead of godoc.org
README: Point to pkg.go.dev instead of godoc.org

The former is the new recommended frontend for browsing Go API
documentation online.

Note that pkg.go.dev doesn't offer status badges at this point in
time[1], so until that feature is introduced we're going to use a
custom badge made using shields.io.

[1] https://github.com/golang/go/issues/36982

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

libvirt/libvirt-go-xml