Added plugin language files for Bulgarian language.

git-svn-id: https://plugins.svn.wordpress.org/appointment-booking-calendar/trunk@1046279 b8457f37-d9ea-0310-8a92-e5e31aec5664
Added info about Bulgarian language.

git-svn-id: https://plugins.svn.wordpress.org/appointment-booking-calendar/trunk@1046303 b8457f37-d9ea-0310-8a92-e5e31aec5664