Add ZenFS to plugin list (facebook#8218)

Summary:
Add ZenFS, a file system for zoned block devices, to PLUGINS.md

Pull Request resolved: facebook#8218

Reviewed By: jay-zhuang

Differential Revision: D27944376

Pulled By: ajkr

fbshipit-source-id: c9ea2e9814001ccd7c56d7ef4d38e20dfeb48d1e

PLUGINS.md

@@ -1,3 +1,4 @@
 This is the list of all known third-party plugins for RocksDB. If something is missing, please open a pull request to add it.
 
 * [Dedupfs](https://github.com/ajkr/dedupfs): an example for plugin developers to reference
+* [ZenFS](https://github.com/westerndigitalcorporation/zenfs): a file system for zoned block devices