message: doc: add macOS DMG opening note to release notes
diff: @@ -43,6 +43,10 @@ built using Qt 5.9.x, which doesn't support versions of macOS older than
  10.10. Additionally, Bitcoin Core does not yet change appearance when
  macOS "dark mode" is activated.
  +Users running macOS Catalina may need to "right-click" and then choose "Open"
  +to open the Bitcoin Core .dmg. This is due to new signing requirements
  +imposed by Apple, which the Bitcoin Core project does not yet adhere too.
  +
  Notable changes
  ===============
  