message: Add reference to PR in CHANGELOG
diff: * Enable encryption on watchOS.
  Cocoa issue [#2876](https://github.com/realm/realm-cocoa/issues/2876).
  + PR [#2598](https://github.com/realm/realm-core/pull/2598).
  * Enforce consistent use of encryption keys across all threads.
  PR [#2558](https://github.com/realm/realm-core/pull/2558).
  