add note on new-merge errors in 0.171 changelog
add note on new-merge errors in 0.171 changelog

Summary: I'll copy this to the github release page as well.

Reviewed By: gkz

Differential Revision: D34110268

fbshipit-source-id: 1f9cf0bec7faeb641383417a9d4ae3a531cd4fe5

facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow