Add pmem-rocksdb-plugin link in PLUGINs.md (facebook#9934)

Summary:
This change adds pmem-rocksdb-plugin link in PLUGINS.md. The link is: https://github.com/pmem/pmem-rocksdb-plugin. It provides a collection plugins to enable Persistent Memory (PMEM) on RocksDB.

The pmem-rocksdb-plugin repo contains RocksDB’s plugins for LSM-tree based KV store to fit it on the PMEM by effectively utilize its characteristics. The first two basic plugins are:
1) Providing a filesystem API wrapper to write RocksDB's WAL (Write Ahead Log) files on PMEM to optimize write performance. 2) Using PMEM as secondary cache to optimize read performance.

Pull Request resolved: facebook#9934

Reviewed By: jay-zhuang

Differential Revision: D36366893

Pulled By: riversand963

fbshipit-source-id: d58a39365e9b5d6a3249d4e9b377c7fb2c79badb

Author: xiao1chen

PLUGINS.md

@@ -4,3 +4,4 @@ This is the list of all known third-party plugins for RocksDB. If something is m
 * [HDFS](https://github.com/riversand963/rocksdb-hdfs-env): an Env used for interacting with HDFS. Migrated from main RocksDB repo
 * [ZenFS](https://github.com/westerndigitalcorporation/zenfs): a file system for zoned block devices
 * [RADOS](https://github.com/riversand963/rocksdb-rados-env): an Env used for interacting with RADOS. Migrated from RocksDB main repo.
+* [PMEM](https://github.com/pmem/pmem-rocksdb-plugin): a collection of plugins to enable Persistent Memory on RocksDB.