message: Add a change log entry for decade helpers [skip ci]
diff: @@ -151,6 +151,16 @@ for the list of changes made since `v2.0.0-alpha.1`.
  - [Added `getUnixTime` function](https://github.com/date-fns/date-fns/pull/870). Kudos to [@Kingwl](https://github.com/Kingwl).
  +- [New decade helpers](https://github.com/date-fns/date-fns/pull/839). Thanks to [@y-nk](https://github.com/y-nk)!
  +
  + - `getDecade`
  +
  + - `startOfDecade`
  +
  + - `endOfDecade`
  +
  + - `lastDayOfDecade`
  +
  ### Changed
  - **BREAKING**: new format string API for `format` function
  