Document the release + contributing process

[joshtrichards]

Presumably either in the README or a Wiki page (or a bit of page)

• Builds and release process
  • New base images (automatic via Docker's Official Images program I believe)
  • Changes to the image itself (e.g. entrypoint.sh, configs, Dockerfile)
  • New Nextcloud Server versions
  • The build pipeline from this repo and through Docker's Official Image program
  • Tests ran against changes via Docker's Official Image program
  • Special situations
    • Security releases
    • Critical image bug releases
• Contributing
  • How to make changes to the images
  • What to commit
  • Tests that run against PRs

[joshtrichards]

Duplicate of #905