Add changelog and readme to bbb plugin to indicate table structure change for sessions

plugin/bbb/README.md

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+BigBlueButton Chamilo plugin
+============================
+This plugin allows you to have videoconference rooms in each course.
+It requires you to have a BigBlueButton videoconference server installed on another server (ideally).
+Check http://www.bigbluebutton.org/ for more about BigBlueButton.

plugin/bbb/changelog.md

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+version 0.9
+-----------
+Released with: Chamilo LMS 1.9.8
+Changes:
+* now supports sessions (requires you to "alter table plugin_bbb_meeting add column session_id int default 0;")
+
+version 0.8
+-----------
+Released with: Chamilo LMS 1.9.0