Summary:
Added .watchmanconfig file to rocksdb repo. It is currently .gitignored.
This allows to auto sync modified files with watchman when editing them remotely.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5593
Differential Revision: D16363860
Pulled By: elipoz
fbshipit-source-id: 5ae221e21c6c757ceb08877771550d508f773d55
