Add Serendipity blogging platform

README.md

@@ -196,6 +196,7 @@ _See also [Static Site Generators](#static-site-generators), [Content Management
 - [Known](https://withknown.com/) - Single website for all your content. ([Source Code](https://github.com/idno/idno)) `Apache-2.0` `PHP`
 - [Plume](https://joinplu.me/) - Federated blogging engine, based on ActivityPub. ([Source Code](https://github.com/Plume-org/Plume)) `AGPL-3.0` `Rust`
 - [PluXml](http://www.pluxml.org/) - XML-based blog/CMS platform. ([Source Code](https://github.com/pluxml/PluXml)) `GPL-1.0` `PHP`
+- [Serendipity](https://docs.s9y.org/) - Serendipity (s9y) is a highly extensible and customizable PHP blog engine using Smarty templating. ([Source Code](https://github.com/s9y/serendipity)) `BSD-3-Clause` `PHP`
 
 
 ## Bookmarks and Link Sharing