editorconfig: set emacs mode

editorconfig: set emacs mode

Some time ago, I proposed to use an (eval) in .dir-locals.el to set
the mode for all json files and Makefile. Unfortunately, this isn't
safe, and emacs will prompt the user, which isn't very friendly.

Fortunately, editorconfig provides a special config key which does
allow to set the emacs mode. Add a few missing entries and set the
emacs mode.

Update top comment to provide a short summary about the file and the
IDE plugins while at it.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>