README: Promote Golang DNS Server
README: Promote Golang DNS Server

The Golang DNS server is no longer experimental; the sslip.io
nameservers run the Golang DNS server.

- Include a placeholder for the `ginkgo` tests to run in CI.
- Include `src/` and `bosh-release/` in the list of directories.
- Mark the PowerDNS-related assets as deprecated; we no longer use
  PowerDNS.

cunnie/sslip.io,cunnie/sslip.io,cunnie/sslip.io,cunnie/sslip.io