Added .watchmanconfig file to rocksdb repo (#5593)

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

.gitignore

@@ -24,6 +24,7 @@ make_config.mk
 *.vcxproj.filters
 *.sln
 *.cmake
+.watchmanconfig
 CMakeCache.txt
 CMakeFiles/
 build/

.watchmanconfig

+{
+  "content_hash_warming": true,
+  "content_hash_max_items": 333333,
+  "hint_num_files_per_dir": 8,
+  "fsevents_latency": 0.05
+}