new users should be warned about cson

improper trailing and leading spaces can cause your bindings to fail.Since cson has very little adoption relative to json, I think it's important to warn new users that they need to understand CoffeeScript notation or else a single space could break their file. I spent one full hour today trying to get an auto-indent mapping to work. The solution involved adding one leading space!
Update keymap.cson
Update keymap.cson
Merge pull request #3769 from Maxhodges/keymap-advice

Warn new users we are using a little known format (cson), not the more popular json doc notation.