0.3

Add DNS-based discovery detection; bump to 0.3.0

- Detect a carbon-txt-location DNS TXT delegation record on the site's
  domain (checking both the apex and www variant) and warn that it
  takes priority over this plugin's output.
- Check the well-known location for an existing file too, with the
  same import tools already offered for the domain root; once
  imported and saved, the old file is permanently deleted instead of
  renamed aside.
- Add a "Keep a copy" section to export disclosures (copy to
  clipboard or download) before deactivating or deleting the plugin.
- Document all v0.3.0 changes in README.md and readme.txt.