Add MongooseIM (#927)

* License, desc for MongooseIM

MongooseIM used to be a fork, it is now a complete, exhaustive platform with many server-side and client-side components: check out the "MongooseIM platform components" https://mongooseim.readthedocs.io/en/latest/

Also, the license of the XMPP server component is GPL, like ejabberd obviously.

* Update README.md

More descriptive description. Update license to GPLv2.

README.md

@@ -384,7 +384,7 @@ _[Extensible Messaging and Presence Protocol](https://en.wikipedia.org/wiki/XMPP
   * [ejabberd](https://www.ejabberd.im/) - XMPP instant messaging server. ([Source Code](https://github.com/processone/ejabberd)) `GPL` `Erlang`
   * [Kontalk](http://kontalk.org/) - Kontalk is an Open Source Messenger, similar to WhatsApp (app for android only currently), including end-to-end encryption, server is based on Tigase XMPP Server. ([Source Code](https://github.com/kontalk)) `GPLv3` `Java`
   * [Metronome IM](https://lightwitch.org/metronome) - Fork of Prosody IM. ([Source Code](http://code.lightwitch.org/)) `MIT` `Lua`
-  * [MongooseIM](https://www.erlang-solutions.com/products/mongooseim.html) - Fork of ejabberd. ([Source Code](https://github.com/esl/MongooseIM)) `MIT` `Erlang`
+  * [MongooseIM](https://www.erlang-solutions.com/products/mongooseim.html) - Mobile messaging platform with a focus on performance and scalability. ([Source Code](https://github.com/esl/MongooseIM)) `GPLv2` `Erlang`
   * [Openfire](http://www.igniterealtime.org/projects/openfire/) - Real time collaboration (RTC) server. ([Source Code](https://github.com/igniterealtime/Openfire)) `Apache` `Java`
   * [Prosody IM](http://prosody.im/) - Feature-rich and easy to configure XMPP server. ([Source Code](http://hg.prosody.im/)) `MIT` `Lua`
   * [Tigase](http://www.tigase.net/content/tigase-xmpp-server) - XMPP server implementation in Java. `GPLv3` `Java`