add BigBlueButton (#1575)

README.md

@@ -482,6 +482,7 @@ _[Extensible Messaging and Presence Protocol](https://en.wikipedia.org/wiki/XMPP
 
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+- [BigBlueButton](https://bigbluebutton.org/) - Supports real-time sharing of audio, video, slides (with whiteboard controls), chat, and the screen. Instructors can engage remote students with polling, emojis, and breakout rooms. ([Demo](https://demo.bigbluebutton.org/gl), [Source Code](https://github.com/bigbluebutton/bigbluebutton)) `LGPL-3.0` `Java`
 - [Conference Organizing Distribution (COD)](http://usecod.com/) - Create conference and event websites built on top of Drupal. ([Source Code](http://cgit.drupalcode.org/cod)) `GPL-1.0` `PHP`
 - [frab](https://frab.github.io/frab/) - web-based conference planning and management system. It helps to collect submissions, to manage talks and speakers and to create a schedule. ([Source Code](https://github.com/frab/frab)) `MIT` `Ruby`
 - [Open Conference Systems (OCS)](https://pkp.sfu.ca/ocs/) - is a free Web publishing tool that will create a complete Web presence for your scholarly conference. ([Demo](https://pkp.sfu.ca/ocs/ocs_demo/), [Source Code](https://github.com/pkp/ocs)) `GPL-1.0` `PHP`