finagle-core: Update changelog

Update changelog to reflect changes made in D1189064: `finagle-core: Add low
priority offload executor`

Differential Revision: https://phabricator.twitter.biz/D1193996

Author: Ivan Gorbachev

CHANGELOG.rst

@@ -31,6 +31,7 @@ New Features
 * finagle-mux: Added a stat, <server_label>/mux/request_context_bytes, that shows the size of the received
   context per request.
 * finagle-netty4: Added support for custom event loop implementations. ``PHAB_ID=D1185136``
+* finagle-core: Add low priority offload executor. ``PHAB_ID=D1189064``
 
 
 Bug Fixes