doc: mention all supported plugins

Also, in the extensions mention the required vim plugin

autoload/airline/extensions/coc.vim

@@ -1,3 +1,7 @@
+" MIT License. Copyright (c) 2019 Peng Guanwen et al.
+" vim: et ts=2 sts=2 sw=2
+" Plugin: https://github.com/neoclide/coc
+
 scriptencoding utf-8
 
 let s:error_symbol = get(g:, 'airline#extensions#coc#error_symbol', 'E:')