comments about the BlockBasedTableOptions migration in Options

Summary: as title

Test Plan: none

Reviewers: sdong, igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22737
update HISTORY.md

Summary: as title

Test Plan: no

Reviewers: igor, sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22761