Add a comment on ContentAllocator changes to CHANGELOG
Add a comment on ContentAllocator changes to CHANGELOG

Summary: TSIA

Differential Revision: D37851117

fbshipit-source-id: 100a5a8f39f5e03be04a4255261ef5ae0fc1f531

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho