Add Rocksdb License to LICENSE file

[sijie]

FEATURE REQUEST

1. Please describe the feature you are requesting.

Rocksdb was introduced for db ledger storage. We need to update our LICENSE file.

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have). Are you currently using any workarounds to address this issue?

blocker for 4.7

[sijie]

This is already addressed by #888