Added OSSN (#538)

* Added OSSN

Added Open Source Social Network.

* Changed ' to `

README.md

@@ -309,6 +309,7 @@ _Mass mailing software - one message to many recipients_
   * [MyBB](http://www.mybb.com/) - Free, extensible forum software package. ([Source Code](https://github.com/mybb/mybb)) `LGPLv3` `PHP`
   * [Newebe](http://newebe.org/) - A Distributed Social Network. ([Source Code](https://github.com/gelnior/newebe)) `AGPLv3` `Python`
   * [NodeBB](https://nodebb.org/) - Node.js based forum software built for the modern web. ([Source Code](https://github.com/NodeBB/NodeBB)) `GPLv3` `Nodejs`
+  * [OSSN](https://www.opensource-socialnetwork.org/) - Open Source Social Network (OSSN) is a social networking software written in PHP. It allows you to make a social networking website and helps your members build social relationships, with people who share similar professional or personal interests. ([Source Code](https://github.com/opensource-socialnetwork/opensource-socialnetwork)) `GPLv2` `PHP`
   * [Oxwall](http://www.oxwall.org/) Oxwall is used for a wide range of projects starting from family sites and custom social networks to collaboration tools and enterprise community solutions. ([Source Code](https://bitbucket.org/oxwall/public) `CPALv1` `PHP`
   * [phpBB](https://www.phpbb.com/) - Flat-forum bulletin board software solution that can be used to stay in touch with a group of people or can power your entire website. ([Source Code](https://github.com/phpbb/phpbb)) `GPLv2` `PHP`
   * [PPnet](https://github.com/pixelpark/ppnet) Create and host your own social network. `MIT` `JavaScript`