Add Mastodon (#971)

* Add Mastodon

* Fix syntax error

README.md

@@ -364,6 +364,7 @@ _[IPBX](https://en.wikipedia.org/wiki/IP_PBX) telephony software_
   * [Isso](http://posativ.org/isso/) - A lightweight commenting server written in Python and Javascript. It aims to be a drop-in replacement for Disqus. ([Source Code](https://github.com/posativ/isso)) `MIT` `Python`
   * [Jappix](https://jappix.com/) - Jappix is an open social platform, that let's you easily get or keep in touch with everyone. ([Source Code](https://github.com/jappix/jappix)) `AGPLv3` `PHP`
   * [Loomio](https://www.loomio.org/) - Loomio is a collaborative decision-making tool that makes it easy for anyone to participate in decisions which affect them. ([Source Code](https://github.com/loomio/loomio)) `AGPLv3` `Ruby`
+  * [Mastodon](https://mastodon.social) - Federated microblogging server, an alternative to GNU social. ([Source Code](https://github.com/tootsuite/mastodon)) `AGPLv3` `Ruby`
   * [Movim](https://movim.eu/) - A brand new social network, full of awesome features in a modern and smart interface. ([Source Code](https://github.com/movim/movim)) `AGPLv3` `PHP`
   * [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`