Document update-types feature within the allow block

[yeikel]

See dependabot/dependabot-core#12668

Example usage

allow:
  - dependency-name: "activesupport"
    update-types: ["version-update:semver-minor", "version-update:semver-patch"]
  - dependency-name: "rack"
    update-types: ["version-update:semver-patch"]
  - dependency-name: "zeitwerk"    # no update-types = allow all
  - dependency-type: "production"
    update-types: ["version-update:semver-patch"]

[github-actions]

Thanks for the PR!

This section of the codebase is owned by @madskristensen and @hyperupcall - if they write a comment saying "LGTM" then it will be merged.

[hyperupcall]

Thanks