specs-go/config.go: Make Root.Readonly omitempty
specs-go/config.go: Make Root.Readonly omitempty

It's an optional setting, and this change will fix the wordy:

  $ ocitools generate --template <(echo '{}')
  $ grep readonly config.json
      "readonly": false,

Instead, config.json will not contain a 'readonly' entry at all.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

ujuc/specs,laijs/specs,hqhq/specs,opencontainers/specs,jonboulle/specs,vbatts/specs,wking/opencontainer-runtime-spec,hqhq/runtime-spec,opencontainers/runtime-spec,wking/opencontainer-specs,cloudfoundry-incubator/specs