message: age-core: Add missing entry for age_core::plugin module
diff: @@ -9,6 +9,9 @@ to 1.0.0 are beta releases.
  ## [Unreleased]
  ### Added
  - `age_core::format::FILE_KEY_BYTES` constant.
  +- `age_core::plugin` module, which contains common backend logic used by both
  + the `age` library (to implement client support for plugins) and the
  + `age-plugin` library.
  ### Changed
  - The stanza prefix `-> ` and trailing newline are now formal parts of the age
  