Add new entry to client-libraries.mdx

A PHP script created for automating Cachet V2.x status and metrics update.

Author: successtar

v2.x/client-libraries.mdx

@@ -21,3 +21,4 @@ If you've made any libraries that consume Cachet's library please let us know by
 | PHP        | https://github.com/checkitonus/php-cachet-api   | CheckItOn.us        |
 | C#         | https://github.com/BerkanYildiz/Cachet.NET      | Berkan Yildiz       |
 | Python     | https://github.com/ZettaIO/cachet-client        | Einar Forselv       |
+| PHP        | https://github.com/successtar/cachet-manager    | Hammed Osanyinpeju  |