Add LogDevice to USERS.md
Add LogDevice to USERS.md

Summary: Closes https://github.com/facebook/rocksdb/pull/2927

Differential Revision: D5906613

Pulled By: siying

fbshipit-source-id: 607401e05b27508c816c700864fe81514606e4ef

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