Add documentation license
Add documentation license

Summary: Closes https://github.com/facebook/rocksdb/pull/2224

Differential Revision: D4966524

Pulled By: siying

fbshipit-source-id: 99f28471bcfc83b0a9df871d5247f62d4a6540ce

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