Update changelog

Author: edgarrmondragon

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Added `logging.LoggerAdapter` to `_MaybeLogger` union https://github.com/python-backoff/backoff/pull/34 (from @jcbertin)
 - Add `exception` to the typing-only `Details` dictionary for cases when `on_exception` is used https://github.com/python-backoff/backoff/pull/35 (from @edgarrmondragon)
 - Add GitHub Actions for CI, documentation, and publishing https://github.com/python-backoff/backoff/pull/39 (from @tysoncung)
+- Use `time.monotonic` instead of `datetime.datetime.now` https://github.com/python-backoff/backoff/pull/23 (from @luccabb)
 
 ### Packaging