update history for 5.6
update history for 5.6

Summary:
- mention range deletion + file ingestion
- move post-5.6 stuff into new section
Closes https://github.com/facebook/rocksdb/pull/2440

Differential Revision: D5229910

Pulled By: ajkr

fbshipit-source-id: 1facfe41993fa1f3b1f6fa7dc77d2b11aa2b317a

bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB